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_data/src/lib.rs

@@ -1,5 +1,5 @@
 use async_trait::async_trait;
-use solitaire_sync::{SyncPayload, SyncResponse};
+use solitaire_sync::{LeaderboardEntry, SyncPayload, SyncResponse};
 use thiserror::Error;
 
 /// All errors that can arise during sync operations.
@@ -33,6 +33,11 @@ pub trait SyncProvider: Send + Sync {
     async fn mirror_achievement(&self, _id: &str) -> Result<(), SyncError> {
         Ok(())
     }
+    /// Fetch the global leaderboard from this backend. Returns an empty list
+    /// for backends that do not support leaderboards (e.g. `LocalOnlyProvider`).
+    async fn fetch_leaderboard(&self) -> Result<Vec<LeaderboardEntry>, SyncError> {
+        Ok(vec![])
+    }
 }
 
 /// Blanket impl so `Box<dyn SyncProvider + Send + Sync>` (returned by
@@ -54,6 +59,9 @@ impl SyncProvider for Box<dyn SyncProvider + Send + Sync> {
     async fn mirror_achievement(&self, id: &str) -> Result<(), SyncError> {
         (**self).mirror_achievement(id).await
     }
+    async fn fetch_leaderboard(&self) -> Result<Vec<LeaderboardEntry>, SyncError> {
+        (**self).fetch_leaderboard().await
+    }
 }
 
 pub mod stats;