feat(core,gui): keep pages up while they reload

Pages no longer swap their content for a "Loading…" status on every load.
A new Loader keeps the last rows on screen and only shows the loading
state if the reply takes longer than 300 ms; a newer load always
supersedes an older one, so a late reply is dropped instead of painted.
Reloading what is on screen keeps it usable; loading something else greys
the old rows out and clears the selection, since their paths would
resolve against the new folder.

My files, search results and Trash now swap only the rows that changed,
so selection, scroll position and thumbnails survive a refresh. Activity
is one sectioned ListView: open conflicts first, then a section per day,
with the filter bar pinned above the list.

Polls give up after 5 s instead of 120 s. A daemon that is running but
doesn't answer now reads as "Not responding" in the app and the tray
rather than as disconnected.

crates/pdfs-core/src/control.rs

@@ -31,6 +31,13 @@ const WRITE_TIMEOUT: Duration = Duration::from_secs(2);
 /// answers.
 const READ_TIMEOUT: Duration = Duration::from_secs(120);
 
+/// Cap on the read half for the requests front ends poll ([`Request::is_poll`]).
+/// The daemon answers those from memory or its database, so a reply that takes
+/// longer than this means it is stuck. Waiting the full [`READ_TIMEOUT`] would
+/// freeze the poller's read-out for two minutes; failing fast lets it say "not
+/// responding" and ask again on its next tick.
+const POLL_READ_TIMEOUT: Duration = Duration::from_secs(5);
+
 /// A command sent from the CLI to the daemon.
 #[derive(Serialize, Deserialize, Debug)]
 pub enum Request {
@@ -739,7 +746,7 @@ pub struct DeviceInfo {
 
 /// One `(sync-conflict …)` copy and the file it is a copy of (in
 /// [`Response::Conflicts`]).
-#[derive(Serialize, Deserialize, Debug, Clone)]
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq)]
 pub struct ConflictInfo {
     /// Mountpoint-relative path of the copy, or its absolute local path when it
     /// sits in a synced folder.
@@ -1025,7 +1032,7 @@ pub enum ActivityKind {
 /// One line in the daemon's activity log (in [`Response::Activity`]). Newest
 /// first. Records a mutation or transfer the daemon performed, with enough
 /// context to read as a sentence: "Uploaded report.pdf to /docs".
-#[derive(Serialize, Deserialize, Debug, Clone)]
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq)]
 pub struct ActivityEntry {
     /// When it happened, epoch seconds.
     pub time: i64,
@@ -1083,7 +1090,7 @@ pub struct TransferItem {
 }
 
 /// One entry in a [`Request::ListDir`] listing.
-#[derive(Serialize, Deserialize, Debug, Clone)]
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq)]
 pub struct DirEntry {
     /// Decrypted node name.
     pub name: String,
@@ -2046,13 +2053,45 @@ impl ErrorKind {
     }
 }
 
+impl Request {
+    /// Whether this is one of the cheap state reads that front ends repeat on a
+    /// timer. The daemon answers these without touching the network, so they
+    /// get the short [`POLL_READ_TIMEOUT`].
+    pub fn is_poll(&self) -> bool {
+        matches!(
+            self,
+            Request::Status
+                | Request::GetQueueStatus
+                | Request::ListLocations
+                | Request::ListPendingOps
+                | Request::ListConflicts
+                | Request::ListActivity { .. }
+                | Request::ThumbnailBuildStatus
+                | Request::PhotosRefreshStatus
+                | Request::RedateStatus
+                | Request::ImportStatus
+        )
+    }
+
+    /// How long to wait for the daemon's reply to this request.
+    pub fn read_timeout(&self) -> Duration {
+        if self.is_poll() {
+            POLL_READ_TIMEOUT
+        } else {
+            READ_TIMEOUT
+        }
+    }
+}
+
 /// Send one [`Request`] to the daemon listening on `socket` and read its
-/// [`Response`]. Returns a crate [`crate::Error`] if no daemon is listening.
+/// [`Response`]. Returns a crate [`crate::Error`] if no daemon is listening, and
+/// one for which [`crate::Error::is_timeout`] holds if the daemon took longer
+/// than [`Request::read_timeout`] to answer.
 ///
 /// Shared by the CLI and GUI so both speak the wire format identically.
 pub fn send(socket: &Path, req: &Request) -> Result<Response> {
     let mut stream = UnixStream::connect(socket)?;
-    stream.set_read_timeout(Some(READ_TIMEOUT))?;
+    stream.set_read_timeout(Some(req.read_timeout()))?;
     stream.set_write_timeout(Some(WRITE_TIMEOUT))?;
     let mut line = serde_json::to_vec(req)?;
     line.push(b'\n');
@@ -2068,6 +2107,36 @@ pub fn send(socket: &Path, req: &Request) -> Result<Response> {
 mod tests {
     use super::*;
 
+    #[test]
+    fn polls_wait_briefly_and_real_work_waits_long() {
+        assert_eq!(Request::Status.read_timeout(), POLL_READ_TIMEOUT);
+        assert_eq!(
+            Request::ListActivity { limit: 200 }.read_timeout(),
+            POLL_READ_TIMEOUT
+        );
+        assert_eq!(
+            Request::OpenFile {
+                path: "a.txt".into(),
+                uid: None,
+            }
+            .read_timeout(),
+            READ_TIMEOUT
+        );
+    }
+
+    #[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()));
+        let _ = std::fs::remove_file(&socket);
+        // Bound but never accepted: connect succeeds, no reply ever comes.
+        let _listener = std::os::unix::net::UnixListener::bind(&socket).unwrap();
+        let started = std::time::Instant::now();
+        let err = send(&socket, &Request::Status).unwrap_err();
+        let _ = std::fs::remove_file(&socket);
+        assert!(err.is_timeout(), "{err}");
+        assert!(started.elapsed() < READ_TIMEOUT);
+    }
+
     #[test]
     fn photo_kind_classifies_by_extension_then_mime() {
         use PhotoKind::*;