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_engine/src/lib.rs

@@ -40,6 +40,7 @@ pub mod selection_plugin;
 pub mod splash_plugin;
 pub mod stats_plugin;
 pub mod sync_plugin;
+pub mod sync_setup_plugin;
 pub mod table_plugin;
 pub mod theme;
 pub mod time_attack_plugin;
@@ -150,6 +151,7 @@ pub use stats_plugin::{
     StatsScreen, StatsUpdate, WatchReplayButton,
 };
 pub use sync_plugin::{SyncPlugin, SyncProviderResource};
+pub use sync_setup_plugin::SyncSetupPlugin;
 pub use ui_focus::{Disabled, FocusGroup, Focusable, FocusedButton, UiFocusPlugin};
 pub use ui_modal::{
     spawn_modal, spawn_modal_actions, spawn_modal_body_text, spawn_modal_button,