feat(core,fuse,gui): push daemon events, polish Photos, adopt newer widgets

The app and the tray stop polling. pdfs-core adds Request::Subscribe: the
connection stays open and carries Event lines (status, transfers, and
Changed with a Topic), with a heartbeat while quiet. In pdfs-fuse, an
EventHub gives each subscriber a mailbox, code that changes something
publishes its topic, and a sampler publishes the status, transfers and sync
progress when they differ. The app follows the feed on one thread, paints
status and transfers as they arrive, and reloads the page on screen when its
topic changes, including after changes made with the pdfs CLI. A hidden
window keeps the changes and catches up when focused. Front-ends fall back
to polling a daemon that predates subscriptions. Refresh buttons leave the
page headers; F5 and Ctrl+R still reload from the server.

Photos gets a scrubber overlaid on the right edge of the timeline in place
of the slider and scrollbar. It shows the years, appears when the pointer
comes near or the timeline scrolls, names the month under the pointer and
jumps to it on drag. Upload becomes a split button whose menu holds the
Google Photos import. Tiles are pooled per row and rebound instead of
rebuilt, hover scales the thumbnail to 1.02 without a shadow, and tiles
lose their tooltip.

On libadwaita 1.8 or newer the app uses AdwSpinner, AdwToggleGroup,
AdwButtonRow and AdwShortcutsDialog, and keeps look-alike stand-ins on
older releases (src/compat.rs, widgets/compat.rs). It still builds against
libadwaita 1.5 and picks at run time, creating the newer widgets by class
name through GtkBuilder, so every package gets them. PDFS_STAND_IN_WIDGETS
forces the stand-ins.

The Sync and share-link icons draw in full again: GTK drops group
transforms from symbolic icons, so their coordinates are baked into the
paths. All catalogs are updated and complete.

crates/pdfs-core/src/control.rs

@@ -679,8 +679,88 @@ pub enum Request {
     /// passes, …), so a front-end can show a running "what happened" feed
     /// without re-deriving it from anywhere.
     ListActivity { limit: usize },
+
+    // ---- push -------------------------------------------------------------
+    /// Keep the connection open and stream [`Event`] lines instead of one
+    /// [`Response`]. See [`subscribe`] for the client side.
+    Subscribe,
 }
 
+/// Something a front-end shows that the daemon can say has changed, so the
+/// front-end reads it again. Carried by [`Event::Changed`].
+#[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[serde(rename_all = "snake_case")]
+pub enum Topic {
+    /// Folder listings in the mount: something was created, renamed, moved,
+    /// trashed or pinned, here or on another device.
+    Files,
+    /// The photos timeline or albums.
+    Photos,
+    Trash,
+    /// Shares, links, invitations and saved links, in either direction.
+    Shares,
+    /// The account's computers.
+    Devices,
+    /// Synced folders, their mode and their live progress.
+    Locations,
+    /// The pending-op queue ([`Request::ListPendingOps`]).
+    Queue,
+    Conflicts,
+    /// The activity log gained entries.
+    Activity,
+    /// A topic from a newer daemon, which this front-end cannot act on.
+    #[serde(other)]
+    Unknown,
+}
+
+impl Topic {
+    /// Every topic a front-end can act on. A new subscription is told all of
+    /// them have changed, so it catches up on whatever happened while it was
+    /// not connected.
+    pub const ALL: [Topic; 9] = [
+        Topic::Files,
+        Topic::Photos,
+        Topic::Trash,
+        Topic::Shares,
+        Topic::Devices,
+        Topic::Locations,
+        Topic::Queue,
+        Topic::Conflicts,
+        Topic::Activity,
+    ];
+}
+
+/// One line on a [`Request::Subscribe`] connection.
+///
+/// The two things front ends show continuously, status and transfers, come
+/// with their contents, so following them costs no requests. Everything else is
+/// a [`Topic`] to re-read.
+#[derive(Serialize, Deserialize, Debug)]
+pub enum Event {
+    /// A fresh [`Response::Status`]. Sent when a subscription starts and
+    /// whenever any of its fields changes.
+    Status(Box<Response>),
+    /// A fresh [`Response::Transfers`] snapshot. Sent when a subscription
+    /// starts and whenever the snapshot changes, which is continuously while
+    /// something moves.
+    Transfers {
+        items: Vec<TransferItem>,
+        jobs: Vec<JobItem>,
+    },
+    /// These topics changed. Changes close together arrive as one event.
+    Changed { topics: Vec<Topic> },
+    /// Nothing happened. Sent every [`HEARTBEAT`] so a client can tell a quiet
+    /// daemon from a stuck one.
+    Heartbeat,
+}
+
+/// How often a quiet subscription sends [`Event::Heartbeat`].
+pub const HEARTBEAT: Duration = Duration::from_secs(10);
+
+/// How long [`Subscription::next_event`] waits for a line before it gives the
+/// daemon up as stuck: a few missed heartbeats.
+const SUBSCRIBE_READ_TIMEOUT: Duration = Duration::from_secs(25);
+
 /// Which kind of share entry a [`ShareEntry`] is, and which collection an
 /// [`Request::UpdateShareRole`]/[`Request::RemoveShareEntry`] targets.
 #[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq)]
@@ -2116,6 +2196,145 @@ pub fn send(socket: &Path, req: &Request) -> Result<Response> {
     Ok(serde_json::from_str(resp.trim())?)
 }
 
+/// An open [`Request::Subscribe`] connection. See [`subscribe`].
+pub struct Subscription {
+    reader: BufReader<UnixStream>,
+    line: String,
+}
+
+/// Start following the daemon listening on `socket`. Fails if no daemon is
+/// listening. A daemon too old to know [`Request::Subscribe`] answers it with an
+/// error, which the first [`Subscription::next_event`] returns as
+/// [`crate::Error::Other`].
+pub fn subscribe(socket: &Path) -> Result<Subscription> {
+    let mut stream = UnixStream::connect(socket)?;
+    stream.set_read_timeout(Some(SUBSCRIBE_READ_TIMEOUT))?;
+    stream.set_write_timeout(Some(WRITE_TIMEOUT))?;
+    let mut line = serde_json::to_vec(&Request::Subscribe)?;
+    line.push(b'\n');
+    stream.write_all(&line)?;
+    stream.flush()?;
+    Ok(Subscription {
+        reader: BufReader::new(stream),
+        line: String::new(),
+    })
+}
+
+impl Subscription {
+    /// Wait for the next event. `Ok(None)` means the daemon closed the
+    /// connection; a timeout means it went [`SUBSCRIBE_READ_TIMEOUT`] without
+    /// even a heartbeat. A line from a newer daemon that this client cannot
+    /// read is skipped rather than ending the subscription.
+    pub fn next_event(&mut self) -> Result<Option<Event>> {
+        loop {
+            self.line.clear();
+            if self.reader.read_line(&mut self.line)? == 0 {
+                return Ok(None);
+            }
+            let line = self.line.trim();
+            if let Ok(event) = serde_json::from_str::<Event>(line) {
+                return Ok(Some(event));
+            }
+            if let Ok(Response::Error { message, .. }) = serde_json::from_str::<Response>(line) {
+                return Err(crate::Error::Other(message));
+            }
+        }
+    }
+}
+
+/// What [`follow`] reports.
+#[derive(Debug)]
+pub enum Feed {
+    /// A subscription started. The events after it describe the daemon's
+    /// current state, so a front-end catches up from them.
+    Connected,
+    Event(Event),
+    /// The subscription ended: the daemon stopped, or stopped answering.
+    Disconnected,
+}
+
+/// How often [`follow`] polls a daemon that cannot push events.
+const LEGACY_POLL: Duration = Duration::from_secs(3);
+
+/// The longest [`follow`] waits between attempts to reach a daemon.
+const RECONNECT_MAX: Duration = Duration::from_secs(5);
+
+/// Follow the daemon on `socket` for as long as `on` returns true, across
+/// daemon restarts. Blocks, so run it on a thread of its own.
+///
+/// A daemon that predates [`Request::Subscribe`] (still running after an
+/// upgrade, until its service restarts) is polled for its status and transfers
+/// instead, so a front-end sees the same events either way, only later.
+pub fn follow(socket: &Path, mut on: impl FnMut(Feed) -> bool) {
+    let mut connected = false;
+    let mut backoff = Duration::from_secs(1);
+    loop {
+        let ended = match subscribe(socket) {
+            Ok(mut subscription) => {
+                let mut first = true;
+                loop {
+                    match subscription.next_event() {
+                        Ok(Some(event)) => {
+                            if first {
+                                first = false;
+                                connected = true;
+                                backoff = Duration::from_secs(1);
+                                if !on(Feed::Connected) {
+                                    return;
+                                }
+                            }
+                            if !on(Feed::Event(event)) {
+                                return;
+                            }
+                        }
+                        Ok(None) => break Ok(()),
+                        Err(e) => break Err(e),
+                    }
+                }
+            }
+            Err(e) => Err(e),
+        };
+        if let Err(crate::Error::Other(_)) = ended {
+            if !connected {
+                connected = true;
+                if !on(Feed::Connected) {
+                    return;
+                }
+            }
+            if !poll_legacy(socket, &mut on) {
+                return;
+            }
+            std::thread::sleep(LEGACY_POLL);
+            continue;
+        }
+        if connected {
+            connected = false;
+            if !on(Feed::Disconnected) {
+                return;
+            }
+        }
+        std::thread::sleep(backoff);
+        backoff = (backoff * 2).min(RECONNECT_MAX);
+    }
+}
+
+/// One round of [`follow`]'s fallback: the status and transfers a subscription
+/// would have pushed. Returns what `on` returned.
+fn poll_legacy(socket: &Path, on: &mut impl FnMut(Feed) -> bool) -> bool {
+    match send(socket, &Request::Status) {
+        Ok(status @ Response::Status { .. }) => {
+            if !on(Feed::Event(Event::Status(Box::new(status)))) {
+                return false;
+            }
+        }
+        _ => return true,
+    }
+    if let Ok(Response::Transfers { items, jobs }) = send(socket, &Request::GetQueueStatus) {
+        return on(Feed::Event(Event::Transfers { items, jobs }));
+    }
+    true
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -2137,6 +2356,59 @@ mod tests {
         );
     }
 
+    /// A daemon on a fresh socket that reads the subscription request and
+    /// answers with `lines`, then hangs up.
+    fn fake_daemon(name: &str, lines: &'static str) -> std::path::PathBuf {
+        let socket = std::env::temp_dir().join(format!("pdfs-{name}-{}.sock", std::process::id()));
+        let _ = std::fs::remove_file(&socket);
+        let listener = std::os::unix::net::UnixListener::bind(&socket).unwrap();
+        std::thread::spawn(move || {
+            let (stream, _) = listener.accept().unwrap();
+            let mut reader = BufReader::new(stream);
+            let mut request = String::new();
+            reader.read_line(&mut request).unwrap();
+            assert!(matches!(
+                serde_json::from_str::<Request>(request.trim()),
+                Ok(Request::Subscribe)
+            ));
+            reader.get_mut().write_all(lines.as_bytes()).unwrap();
+        });
+        socket
+    }
+
+    #[test]
+    fn a_subscription_skips_what_it_cannot_read() {
+        let socket = fake_daemon(
+            "subscribe",
+            "not json\n\
+             {\"Changed\":{\"topics\":[\"files\",\"from_a_newer_daemon\"]}}\n\
+             {\"NewEvent\":{}}\n\
+             \"Heartbeat\"\n",
+        );
+        let mut subscription = subscribe(&socket).unwrap();
+        let first = subscription.next_event().unwrap();
+        let second = subscription.next_event().unwrap();
+        let end = subscription.next_event().unwrap();
+        let _ = std::fs::remove_file(&socket);
+        match first {
+            Some(Event::Changed { topics }) => {
+                assert_eq!(topics, vec![Topic::Files, Topic::Unknown]);
+            }
+            other => panic!("expected a change, got {other:?}"),
+        }
+        assert!(matches!(second, Some(Event::Heartbeat)), "{second:?}");
+        assert!(end.is_none(), "{end:?}");
+    }
+
+    #[test]
+    fn a_daemon_without_subscriptions_is_told_apart_from_a_dead_one() {
+        let socket = fake_daemon("legacy", "{\"Error\":{\"message\":\"unknown request\"}}\n");
+        let mut subscription = subscribe(&socket).unwrap();
+        let answer = subscription.next_event();
+        let _ = std::fs::remove_file(&socket);
+        assert!(matches!(answer, Err(crate::Error::Other(_))), "{answer:?}");
+    }
+
     #[test]
     fn a_daemon_that_never_answers_times_out_as_not_responding() {
         let socket = std::env::temp_dir().join(format!("pdfs-silent-{}.sock", std::process::id()));