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/events.rs

@@ -129,6 +129,31 @@ pub struct StartTimeAttackRequestEvent;
 #[derive(Message, Debug, Clone, Copy, Default)]
 pub struct StartDailyChallengeRequestEvent;
 
+/// Request to toggle the Stats overlay. Fired by the HUD Menu-popover
+/// "Stats" row alongside the existing `S` accelerator.
+#[derive(Message, Debug, Clone, Copy, Default)]
+pub struct ToggleStatsRequestEvent;
+
+/// Request to toggle the Achievements overlay. Fired by the HUD
+/// Menu-popover "Achievements" row alongside the existing `A` accelerator.
+#[derive(Message, Debug, Clone, Copy, Default)]
+pub struct ToggleAchievementsRequestEvent;
+
+/// Request to toggle the Profile overlay. Fired by the HUD Menu-popover
+/// "Profile" row alongside the existing `P` accelerator.
+#[derive(Message, Debug, Clone, Copy, Default)]
+pub struct ToggleProfileRequestEvent;
+
+/// Request to toggle the Settings overlay. Fired by the HUD Menu-popover
+/// "Settings" row alongside the existing `O` accelerator.
+#[derive(Message, Debug, Clone, Copy, Default)]
+pub struct ToggleSettingsRequestEvent;
+
+/// Request to toggle the Leaderboard overlay. Fired by the HUD
+/// Menu-popover "Leaderboard" row alongside the existing `L` accelerator.
+#[derive(Message, Debug, Clone, Copy, Default)]
+pub struct ToggleLeaderboardRequestEvent;
+
 /// Fired by `SyncPlugin` after a pull task resolves and the merged result has
 /// been persisted to disk. `Ok(SyncResponse)` carries the merged payload plus
 /// any `ConflictReport`s the merge produced. `Err(String)` carries a