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/lib.rs

@@ -35,8 +35,8 @@ pub use animation_plugin::{AnimationPlugin, CardAnim};
 pub use audio_plugin::{AudioPlugin, AudioState, SoundLibrary};
 pub use card_plugin::{CardEntity, CardLabel, CardPlugin};
 pub use events::{
-    AchievementUnlockedEvent, CardFlippedEvent, DrawRequestEvent, GameWonEvent, MoveRejectedEvent,
-    MoveRequestEvent, NewGameRequestEvent, StateChangedEvent, UndoRequestEvent,
+    AchievementUnlockedEvent, CardFlippedEvent, DrawRequestEvent, GameWonEvent, ManualSyncRequestEvent,
+    MoveRejectedEvent, MoveRequestEvent, NewGameRequestEvent, StateChangedEvent, UndoRequestEvent,
 };
 pub use game_plugin::{GameMutation, GamePlugin};
 pub use help_plugin::{HelpPlugin, HelpScreen};