feat(engine): add Menu dropdown for Stats/Achievements/Profile/Settings/Leaderboard

Continues the UI-first pass. The five informational overlays were
each behind a single-key shortcut (S/A/P/O/L) with no visible UI
affordance. Add a "Menu ▾" button to the action bar that toggles a
popover with one row per overlay. Each row dispatches the same code
path the keyboard accelerator uses by writing a new
`Toggle*RequestEvent`:

- Stats        → ToggleStatsRequestEvent
- Achievements → ToggleAchievementsRequestEvent
- Profile      → ToggleProfileRequestEvent
- Settings     → ToggleSettingsRequestEvent
- Leaderboard  → ToggleLeaderboardRequestEvent

Each plugin's existing toggle handler now reads either its key or
the matching request event so the spawn / despawn / fetch logic stays
in the owning plugin (the popover never duplicates that behaviour).

Action bar order is now (left → right):
  Menu ▾   Undo   Pause   Help   Modes ▾   New Game

Menu sits on the far left because it's a navigation aggregator;
New Game stays on the far right as the most consequential action.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

solitaire_engine/src/lib.rs

@@ -71,14 +71,16 @@ pub use events::{
     ManualSyncRequestEvent, MoveRejectedEvent, MoveRequestEvent, NewGameConfirmEvent,
     NewGameRequestEvent, PauseRequestEvent, StartChallengeRequestEvent,
     StartDailyChallengeRequestEvent, StartTimeAttackRequestEvent, StartZenRequestEvent,
-    StateChangedEvent, SyncCompleteEvent, UndoRequestEvent, XpAwardedEvent,
+    StateChangedEvent, SyncCompleteEvent, ToggleAchievementsRequestEvent,
+    ToggleLeaderboardRequestEvent, ToggleProfileRequestEvent, ToggleSettingsRequestEvent,
+    ToggleStatsRequestEvent, UndoRequestEvent, XpAwardedEvent,
 };
 pub use game_plugin::{ConfirmNewGameScreen, GameMutation, GameOverScreen, GamePlugin, GameStatePath};
 pub use help_plugin::{HelpPlugin, HelpScreen};
 pub use home_plugin::{HomePlugin, HomeScreen};
 pub use hud_plugin::{
-    ActionButton, HelpButton, HudAutoComplete, HudPlugin, ModeOption, ModesButton, ModesPopover,
-    NewGameButton, PauseButton, UndoButton,
+    ActionButton, HelpButton, HudAutoComplete, HudPlugin, MenuButton, MenuOption, MenuPopover,
+    ModeOption, ModesButton, ModesPopover, NewGameButton, PauseButton, UndoButton,
 };
 pub use leaderboard_plugin::{LeaderboardPlugin, LeaderboardResource, LeaderboardScreen};
 pub use input_plugin::InputPlugin;