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/db/ops.rs

@@ -772,6 +772,31 @@ impl Db {
         Ok(())
     }
 
+    /// Make one backed-off op due now, for the user's "Retry now". Parked ops are
+    /// left alone: they wait for a rename, not for time, and un-parking one would
+    /// upload a file its writer has not finished. Returns whether a row changed.
+    pub fn retry_op_now(&self, id: i64, now: i64) -> Result<bool> {
+        let conn = self.conn.lock();
+        let changed = conn.execute(
+            "UPDATE pending_op SET next_attempt_at = ?2
+             WHERE id = ?1 AND next_attempt_at > ?2 AND next_attempt_at < ?3",
+            params![id, now, PARK_UNTIL],
+        )?;
+        Ok(changed > 0)
+    }
+
+    /// [`retry_op_now`](Self::retry_op_now) for every op that has failed at
+    /// least once. Returns how many rows changed.
+    pub fn retry_failed_ops_now(&self, now: i64) -> Result<usize> {
+        let conn = self.conn.lock();
+        let changed = conn.execute(
+            "UPDATE pending_op SET next_attempt_at = ?1
+             WHERE attempts > 0 AND next_attempt_at > ?1 AND next_attempt_at < ?2",
+            params![now, PARK_UNTIL],
+        )?;
+        Ok(changed)
+    }
+
     /// Record a failed attempt and when to next try. Leaves the row in place —
     /// the staged bytes are still the only copy of the user's write.
     ///