feat(analytics): opt-in usage analytics with server ingest and settings toggle

- Server: POST /api/analytics endpoint with per-IP rate limit (5/min),
  batch validation (≤50 events, event_type regex, UUID dedup, clock check),
  INSERT OR IGNORE for idempotency, and migration 004_analytics.sql
- Client (solitaire_data): AnalyticsClient with in-memory Mutex buffer,
  UUID session_id per launch, async flush via background task
- Engine: AnalyticsPlugin records game_won, game_forfeit, game_start,
  achievement_unlocked; flushes immediately on game-end, every 60 s otherwise
- Settings UI: Privacy section with ON/OFF toggle, hidden in local-only mode
- Default: analytics_enabled = false (explicit opt-in required)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_app/src/lib.rs

@@ -25,7 +25,7 @@ use bevy::window::{Monitor, PrimaryMonitor, PrimaryWindow};
 use bevy::winit::WinitWindows;
 use solitaire_data::{load_settings_from, provider_for_backend, settings_file_path, Settings};
 use solitaire_engine::{
-    register_theme_asset_sources, AchievementPlugin, AnimationPlugin, AssetSourcesPlugin,
+    register_theme_asset_sources, AchievementPlugin, AnalyticsPlugin, AnimationPlugin, AssetSourcesPlugin,
     AudioPlugin, AutoCompletePlugin, CardAnimationPlugin, CardPlugin, ChallengePlugin,
     CursorPlugin, DailyChallengePlugin, DiagnosticsHudPlugin, DifficultyPlugin, FeedbackAnimPlugin,
     FontPlugin, GamePlugin, HelpPlugin, HomePlugin, HudPlugin, InputPlugin, LeaderboardPlugin,
@@ -194,6 +194,7 @@ pub fn run() {
         .add_plugins(OnboardingPlugin)
         .add_plugins(SyncPlugin::new(sync_provider))
         .add_plugins(SyncSetupPlugin)
+        .add_plugins(AnalyticsPlugin)
         .add_plugins(LeaderboardPlugin)
         .add_plugins(WinSummaryPlugin)
         .add_plugins(UiModalPlugin)