feat(engine): leaderboard screen (press L to toggle)

Press L to open a leaderboard overlay. On open, an async fetch is
dispatched against the active SyncProvider. Results cache in
LeaderboardResource; the panel rebuilds live when data arrives.
Closing during a fetch is safe (ClosedThisFrame flag prevents
re-spawning the panel in the same frame as the user's despawn command).
Format: ranked table with player name, best score, and fastest win time.
Non-authenticated / LocalOnly providers return an empty list gracefully.

- solitaire_data: add fetch_leaderboard() to SyncProvider trait
  (default → Ok([])) and implement in SolitaireServerClient
- solitaire_engine: new LeaderboardPlugin with 5 unit tests
- solitaire_app: register LeaderboardPlugin

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

solitaire_app/src/main.rs

@@ -2,9 +2,9 @@ use bevy::prelude::*;
 use solitaire_data::{load_settings_from, provider_for_backend, settings_file_path, Settings};
 use solitaire_engine::{
     AchievementPlugin, AnimationPlugin, AudioPlugin, AutoCompletePlugin, CardPlugin,
-    ChallengePlugin, DailyChallengePlugin, GamePlugin, HelpPlugin, InputPlugin, OnboardingPlugin,
-    PausePlugin, ProgressPlugin, SettingsPlugin, StatsPlugin, SyncPlugin, TablePlugin,
-    TimeAttackPlugin, WeeklyGoalsPlugin,
+    ChallengePlugin, DailyChallengePlugin, GamePlugin, HelpPlugin, InputPlugin, LeaderboardPlugin,
+    OnboardingPlugin, PausePlugin, ProgressPlugin, SettingsPlugin, StatsPlugin, SyncPlugin,
+    TablePlugin, TimeAttackPlugin, WeeklyGoalsPlugin,
 };
 
 fn main() {
@@ -45,5 +45,6 @@ fn main() {
         .add_plugins(AudioPlugin)
         .add_plugins(OnboardingPlugin)
         .add_plugins(SyncPlugin::new(sync_provider))
+        .add_plugins(LeaderboardPlugin)
         .run();
 }