feat(engine): MoveRejectedEvent + PausePlugin (Esc)

- New MoveRejectedEvent fires from end_drag when the cursor is over
  a real pile but the placement is illegal. AudioPlugin plays
  card_invalid.wav on it.
- New PausePlugin + PausedResource: Esc toggles a full-window
  overlay and the flag. tick_elapsed_time and advance_time_attack
  skip work while paused. Help cheat sheet updated.

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

solitaire_app/src/main.rs

@@ -1,8 +1,8 @@
 use bevy::prelude::*;
 use solitaire_engine::{
     AchievementPlugin, AnimationPlugin, AudioPlugin, CardPlugin, ChallengePlugin,
-    DailyChallengePlugin, GamePlugin, HelpPlugin, InputPlugin, ProgressPlugin, StatsPlugin,
-    TablePlugin, TimeAttackPlugin, WeeklyGoalsPlugin,
+    DailyChallengePlugin, GamePlugin, HelpPlugin, InputPlugin, PausePlugin, ProgressPlugin,
+    StatsPlugin, TablePlugin, TimeAttackPlugin, WeeklyGoalsPlugin,
 };
 
 fn main() {
@@ -30,6 +30,7 @@ fn main() {
         .add_plugins(ChallengePlugin)
         .add_plugins(TimeAttackPlugin)
         .add_plugins(HelpPlugin)
+        .add_plugins(PausePlugin)
         .add_plugins(AudioPlugin)
         .run();
 }