feat(engine): wire Stats Watch Replay button to in-engine playback

Promotes the Stats overlay's Watch Replay button from a stub
InfoToastEvent ("playback coming in a future build") to actually
starting in-engine playback via the new
replay_playback::start_replay_playback API. Pressing the button
when a replay exists resets the game to the recorded deal and the
ReplayOverlayPlugin's banner takes over.

The handler reads ReplayPlaybackState via Option<ResMut<...>> so
headless test fixtures that don't register ReplayPlaybackPlugin
keep compiling — they fall back to a descriptive "Replay ready"
toast. The "no replay yet" branch still surfaces the existing
"win a game first" toast.

Plugin registration in solitaire_app/src/main.rs picks up
ReplayPlaybackPlugin and ReplayOverlayPlugin alongside the existing
StatsPlugin. They run in any order — the playback plugin owns the
state resource, the overlay plugin spawns/despawns based on state
changes, and Stats's button handler dispatches into the playback
plugin's API.

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

solitaire_app/src/main.rs

@@ -10,7 +10,8 @@ use solitaire_engine::{
     AudioPlugin, AutoCompletePlugin, CardAnimationPlugin, CardPlugin, ChallengePlugin,
     CursorPlugin, DailyChallengePlugin, FeedbackAnimPlugin, FontPlugin, GamePlugin, HelpPlugin,
     HomePlugin, HudPlugin, InputPlugin, LeaderboardPlugin, OnboardingPlugin, PausePlugin,
-    ProfilePlugin, ProgressPlugin, RadialMenuPlugin, SelectionPlugin, SettingsPlugin, SplashPlugin,
+    ProfilePlugin, ProgressPlugin, RadialMenuPlugin, ReplayOverlayPlugin, ReplayPlaybackPlugin,
+    SelectionPlugin, SettingsPlugin, SplashPlugin,
     StatsPlugin, SyncPlugin, TablePlugin, ThemePlugin, ThemeRegistryPlugin, TimeAttackPlugin,
     UiFocusPlugin, UiModalPlugin, UiTooltipPlugin, WeeklyGoalsPlugin, WinSummaryPlugin,
 };
@@ -117,6 +118,8 @@ fn main() {
         .add_plugins(FeedbackAnimPlugin)
         .add_plugins(CardAnimationPlugin)
         .add_plugins(AutoCompletePlugin)
+        .add_plugins(ReplayPlaybackPlugin)
+        .add_plugins(ReplayOverlayPlugin)
         .add_plugins(StatsPlugin::default())
         .add_plugins(ProgressPlugin::default())
         .add_plugins(AchievementPlugin::default())