feat(core,fuse,gui,cli): pause/resume sync and a visible upload queue (phase 2)

Backend (docs/UI_UX_PLAN.md B-1, B-2):
- Request::SetSyncPaused { paused, until } pauses the drain worker and the
  mirror reconcile. FUSE reads, write staging, the watcher and the poll keep
  running. The pause is persisted in the `state` table (`sync_paused_until`),
  so it survives a restart; a timed pause ends by itself through a timer
  thread, re-armed on mount. Resuming wakes the drain and runs a full
  reconcile. Response::Status gains `paused` and `paused_until`.
- Request::ListPendingOps lists the queue with paths, attempts, last error,
  next attempt and parked/failing flags. Request::RetryPendingOp { id } makes
  one backed-off op (or every failed one) due now. Parked ops are never
  un-parked and nothing is dropped from the queue.

Front ends:
- Sync page: a status card with Pause/Resume (split button with 1 h, 8 h and
  24 h pauses) and a "Waiting to Upload" list with Retry now and Retry All.
- The sidebar strip and the tray show the paused state; the tray gains Pause
  Sync / Resume Sync and names failing changes.
- CLI: `pdfs sync pause [--for 30m|1h|2d]`, `pdfs sync resume`,
  `pdfs sync queue`, `pdfs sync retry [id]`; `pdfs status` reports a pause.

Not in this phase: merging Activity into the Sync page, discarding a queued
op (needs a recovery copy first), and per-folder pause.

crates/pdfs-core/src/control.rs

@@ -384,6 +384,18 @@ pub enum Request {
     /// Force a reconcile pass: one folder by id, or all when `id` is `None`.
     /// Replies with [`Response::Ok`].
     SyncNow { id: Option<i64> },
+    /// Pause or resume syncing: queued uploads and changes stop draining and
+    /// mirror folders stop reconciling, while reads through the mount keep
+    /// working. `until` is the Unix second a pause ends by itself; `None`
+    /// pauses until resumed. Ignored when resuming. Replies with
+    /// [`Response::Ok`].
+    SetSyncPaused { paused: bool, until: Option<i64> },
+    /// List the queue of uploads and changes not yet on the remote. Replies
+    /// with [`Response::PendingOps`].
+    ListPendingOps,
+    /// Retry a backed-off queued op now instead of waiting out its backoff;
+    /// every failed op when `id` is `None`. Replies with [`Response::Ok`].
+    RetryPendingOp { id: Option<i64> },
     /// List the folders under this machine's device that can be synced here,
     /// each with a proposed local path (features.md 5.2). Replies with
     /// [`Response::RestorableFolders`].
@@ -626,6 +638,30 @@ pub struct DeviceInfo {
     pub adopted: bool,
 }
 
+/// One queued upload or change (in [`Response::PendingOps`]).
+#[derive(Serialize, Deserialize, Debug, Clone)]
+pub struct PendingOpInfo {
+    /// Queue row id, for [`Request::RetryPendingOp`].
+    pub id: i64,
+    /// `revision`, `create`, `mkdir`, `rename` or `trash`.
+    pub kind: String,
+    /// Mountpoint-relative path of the file or folder, or its name when the
+    /// path is no longer known.
+    pub path: String,
+    /// Failed attempts so far.
+    pub attempts: i64,
+    /// The latest failure, when there has been one.
+    pub last_error: Option<String>,
+    /// Unix second the op was queued.
+    pub queued_at: i64,
+    /// Unix second of the next attempt; `None` while parked.
+    pub next_attempt_at: Option<i64>,
+    /// Held back until a transient file gets its final name.
+    pub parked: bool,
+    /// Failed often enough to count as stuck.
+    pub failing: bool,
+}
+
 /// One synced local folder on this machine's device (in [`Response::SyncFolders`]).
 #[derive(Serialize, Deserialize, Debug, Clone)]
 pub struct SyncFolderInfo {
@@ -1562,6 +1598,13 @@ pub enum Response {
         /// Age of the oldest staged write, in seconds.
         #[serde(default)]
         staged_oldest_secs: u64,
+        /// True while the user has sync paused (see [`Request::SetSyncPaused`]).
+        #[serde(default)]
+        paused: bool,
+        /// The Unix second a timed pause ends; `None` when paused until
+        /// resumed, or not paused.
+        #[serde(default)]
+        paused_until: Option<i64>,
     },
     /// A human-readable success message.
     Ok { message: String },
@@ -1691,6 +1734,8 @@ pub enum Response {
     Locations { items: Vec<MountSpec> },
     /// This device's synced folders (reply to [`Request::ListSyncFolders`]).
     SyncFolders { items: Vec<SyncFolderInfo> },
+    /// The upload/change queue (reply to [`Request::ListPendingOps`]).
+    PendingOps { items: Vec<PendingOpInfo> },
     /// Folders offered for restore (reply to [`Request::ListRestorableFolders`]).
     RestorableFolders { items: Vec<RestorableFolder> },
     /// A node's share: members + pending invitations, and its public link if any