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_data/src/lib.rs

@@ -163,5 +163,8 @@ pub use replay::{
     REPLAY_HISTORY_CAP, REPLAY_HISTORY_SCHEMA_VERSION, REPLAY_SCHEMA_VERSION,
 };
 
+pub mod analytics_client;
+pub use analytics_client::AnalyticsClient;
+
 pub mod platform;
 pub use platform::data_dir;