feat(data,engine): persistent Settings + SFX volume hotkeys

- solitaire_data::Settings { sfx_volume, first_run_complete } with
  atomic JSON persistence and clamping sanitizer.
- SettingsPlugin (engine): [ / ] adjust SFX volume by 0.1, clamped;
  persists on change; emits SettingsChangedEvent. No-op at rails.
- AudioPlugin applies sfx_volume to kira's main track at startup
  and on every change so live tweaks take effect without restart.
- Brief "SFX: N%" toast on each change. Help cheat sheet updated.

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

solitaire_app/src/main.rs

@@ -2,7 +2,7 @@ use bevy::prelude::*;
 use solitaire_engine::{
     AchievementPlugin, AnimationPlugin, AudioPlugin, CardPlugin, ChallengePlugin,
     DailyChallengePlugin, GamePlugin, HelpPlugin, InputPlugin, PausePlugin, ProgressPlugin,
-    StatsPlugin, TablePlugin, TimeAttackPlugin, WeeklyGoalsPlugin,
+    SettingsPlugin, StatsPlugin, TablePlugin, TimeAttackPlugin, WeeklyGoalsPlugin,
 };
 
 fn main() {
@@ -31,6 +31,7 @@ fn main() {
         .add_plugins(TimeAttackPlugin)
         .add_plugins(HelpPlugin)
         .add_plugins(PausePlugin)
+        .add_plugins(SettingsPlugin::default())
         .add_plugins(AudioPlugin)
         .run();
 }