feat(fuse,core,cli,gui): make folders available offline from the app

The daemon has kept recursive folder pins for a long time, but the app
offered "Available offline" for files only. The context menu, the
details pane and the selection bar now offer it for folders too, and a
pinned folder carries the offline badge in the browser and the launcher.

A folder pin used to download the whole subtree inside the control
request, so a large folder outlasted the client's 120 s read timeout,
and every pinned file was read into memory before it was cached. The
daemon now records the pin, answers at once and downloads on a
background thread as a job the tray and the app show, stopping when the
folder is unpinned or the daemon stops. Pins of files and folders both
go through fetch_content, which streams to disk and skips fresh blobs.
A folder pin is refused while offline instead of looking kept.

A file kept only because a folder above it is pinned has no pin of its
own, and unpinning it silently did nothing. ListDir now reports
pinned_by_folder; the app shows such an entry as available offline with
the toggle greyed out, and the daemon refuses the unpin with a message
naming the folder. Unpinning a folder no longer evicts files that are
pinned on their own.

Requested on Reddit.

crates/pdfs-core/src/control.rs

@@ -1260,8 +1260,15 @@ pub struct DirEntry {
     pub size: u64,
     /// Modification time, epoch seconds.
     pub modified: i64,
-    /// Whether the file is pinned to this device.
+    /// Whether the node is pinned to this device, on its own or through a
+    /// pinned folder above it.
     pub pinned: bool,
+    /// Whether the node is kept on this device because a folder above it is
+    /// pinned. Such a node cannot be unpinned on its own. Set in
+    /// [`Request::ListDir`] listings only. Defaulted for wire-compat with
+    /// daemons predating the field.
+    #[serde(default)]
+    pub pinned_by_folder: bool,
     /// Whether the file's full content is present in the local cache (a current,
     /// non-stale blob). Always false for folders. Defaulted for wire-compat with
     /// clients/daemons predating the field.