feat(workspace): full server + sync implementation, all tests green

- solitaire_server: Axum auth, sync push/pull, leaderboard, daily
  challenge, account deletion, JWT middleware, rate limiting via
  tower_governor, SQLite migrations, health endpoint
- solitaire_server: expose build_test_router (no rate limiting) so
  integration tests work without a peer IP in oneshot requests
- solitaire_sync: SyncPayload, merge logic, shared API types
- solitaire_data: SyncProvider trait, LocalOnlyProvider,
  SolitaireServerClient, auth_tokens keyring integration, blanket
  Box<dyn SyncProvider> impl
- solitaire_data/settings: derive Default on SyncBackend (clippy fix)
- .sqlx/: offline query cache so server compiles without a live DB
- sqlx: removed non-existent "offline" feature flag
- keyring v2: fixed Entry::new() returning Result<Entry>
- sqlx 0.8: all SQLite TEXT columns wrapped in Option<T>
- Integration tests: max_connections(1) on in-memory pool so all
  connections share the same schema

All 191 tests pass; cargo clippy -D warnings clean.

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

solitaire_app/Cargo.toml

@@ -10,3 +10,4 @@ path = "src/main.rs"
 [dependencies]
 bevy             = { workspace = true }
 solitaire_engine = { workspace = true }
+solitaire_data   = { workspace = true }

solitaire_app/src/main.rs

@@ -1,11 +1,20 @@
 use bevy::prelude::*;
+use solitaire_data::{load_settings_from, provider_for_backend, settings_file_path, Settings};
 use solitaire_engine::{
     AchievementPlugin, AnimationPlugin, AudioPlugin, CardPlugin, ChallengePlugin,
     DailyChallengePlugin, GamePlugin, HelpPlugin, InputPlugin, OnboardingPlugin, PausePlugin,
-    ProgressPlugin, SettingsPlugin, StatsPlugin, TablePlugin, TimeAttackPlugin, WeeklyGoalsPlugin,
+    ProgressPlugin, SettingsPlugin, StatsPlugin, SyncPlugin, TablePlugin, TimeAttackPlugin,
+    WeeklyGoalsPlugin,
 };
 
 fn main() {
+    // Load settings before building the app so we can construct the right
+    // sync provider. Falls back to defaults if no settings file exists yet.
+    let settings: Settings = settings_file_path()
+        .map(|p| load_settings_from(&p))
+        .unwrap_or_default();
+    let sync_provider = provider_for_backend(&settings.sync_backend);
+
     App::new()
         .add_plugins(
             DefaultPlugins.set(WindowPlugin {
@@ -34,5 +43,6 @@ fn main() {
         .add_plugins(SettingsPlugin::default())
         .add_plugins(AudioPlugin)
         .add_plugins(OnboardingPlugin)
+        .add_plugins(SyncPlugin::new(sync_provider))
         .run();
 }