feat(engine,core): add Time Attack mode + unlocks panel

- Core: GameMode::TimeAttack variant (no scoring/undo changes — session marker only)
- Engine: TimeAttackPlugin with TimeAttackResource, TimeAttackEndedEvent,
  T hotkey (gated to level >= 5), auto-deal on win, summary toast
- Engine: Stats overlay (S) gains an Unlocks subsection (card backs /
  backgrounds, sorted/deduped) and a live Time Attack panel while active

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

solitaire_app/src/main.rs

@@ -1,7 +1,8 @@
 use bevy::prelude::*;
 use solitaire_engine::{
     AchievementPlugin, AnimationPlugin, CardPlugin, ChallengePlugin, DailyChallengePlugin,
-    GamePlugin, InputPlugin, ProgressPlugin, StatsPlugin, TablePlugin, WeeklyGoalsPlugin,
+    GamePlugin, InputPlugin, ProgressPlugin, StatsPlugin, TablePlugin, TimeAttackPlugin,
+    WeeklyGoalsPlugin,
 };
 
 fn main() {
@@ -27,5 +28,6 @@ fn main() {
         .add_plugins(DailyChallengePlugin)
         .add_plugins(WeeklyGoalsPlugin)
         .add_plugins(ChallengePlugin)
+        .add_plugins(TimeAttackPlugin)
         .run();
 }