feat(engine): ManualSyncRequestEvent + Sync Now button in settings

- Added ManualSyncRequestEvent to events.rs (exported from lib.rs).
- SyncPlugin now handles ManualSyncRequestEvent: if no pull is in
  flight, spawns a new AsyncComputeTaskPool task and sets status to
  Syncing. Ignores duplicate requests while a pull is active.
- Settings panel "Sync" section now shows the status text alongside a
  "Sync Now" button that fires ManualSyncRequestEvent.
- Cleaned up stale doc comment in input_plugin.rs (Esc pause note).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/input_plugin.rs

@@ -4,8 +4,7 @@
 //! - `U` → `UndoRequestEvent`
 //! - `N` → `NewGameRequestEvent { seed: None }`
 //! - `D` → `DrawRequestEvent`
-//! - `Esc` → logged as a pause placeholder (no event yet; wired up when the
-//!   pause screen lands in a later phase)
+//! - `Esc` → handled by `PausePlugin` (overlay toggle + paused flag)
 //!
 //! Mouse:
 //! - Left-click on the stock pile (face-down top) → `DrawRequestEvent`