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_engine/src/lib.rs

@@ -10,6 +10,7 @@ pub mod daily_challenge_plugin;
 pub mod events;
 pub mod game_plugin;
 pub mod help_plugin;
+pub mod leaderboard_plugin;
 pub mod input_plugin;
 pub mod layout;
 pub mod onboarding_plugin;
@@ -42,6 +43,7 @@ pub use events::{
 };
 pub use game_plugin::{GameMutation, GamePlugin, GameStatePath};
 pub use help_plugin::{HelpPlugin, HelpScreen};
+pub use leaderboard_plugin::{LeaderboardPlugin, LeaderboardResource, LeaderboardScreen};
 pub use input_plugin::InputPlugin;
 pub use onboarding_plugin::{OnboardingPlugin, OnboardingScreen};
 pub use pause_plugin::{PausePlugin, PauseScreen, PausedResource};