feat(sync): Phase 8 sync setup UI — login/register modal + Connect/Disconnect

Adds SyncSetupPlugin: a three-field (URL / Username / Password) modal
that handles both login and register flows via an async task on
AsyncComputeTaskPool wrapped in a Tokio single-thread runtime (same
pattern as the existing sync push/pull). On success, tokens are stored
to the OS keychain / Android Keystore and SyncProviderResource is
hot-swapped so subsequent pull/push use the new credentials immediately.

Settings sync section now shows Connect (when Local) or Sync Now +
Disconnect + username label (when SolitaireServer). SyncAuthResultEvent
stub registered for future re-auth prompt wiring.

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

solitaire_app/src/lib.rs

@@ -32,7 +32,7 @@ use solitaire_engine::{
     OnboardingPlugin, PausePlugin, PlayBySeedPlugin, ProfilePlugin, ProgressPlugin,
     RadialMenuPlugin, ReplayOverlayPlugin, ReplayPlaybackPlugin, SafeAreaInsetsPlugin,
     SelectionPlugin, SettingsPlugin,
-    SplashPlugin, StatsPlugin, SyncPlugin, TablePlugin, ThemePlugin, ThemeRegistryPlugin,
+    SplashPlugin, StatsPlugin, SyncPlugin, SyncSetupPlugin, TablePlugin, ThemePlugin, ThemeRegistryPlugin,
     TimeAttackPlugin, UiFocusPlugin, UiModalPlugin, UiTooltipPlugin, WeeklyGoalsPlugin,
     WinSummaryPlugin,
 };
@@ -193,6 +193,7 @@ pub fn run() {
         .add_plugins(AudioPlugin)
         .add_plugins(OnboardingPlugin)
         .add_plugins(SyncPlugin::new(sync_provider))
+        .add_plugins(SyncSetupPlugin)
         .add_plugins(LeaderboardPlugin)
         .add_plugins(WinSummaryPlugin)
         .add_plugins(UiModalPlugin)