feat(leaderboard): opt-out support — server endpoint, client method, UI button

- Server: DELETE /api/leaderboard/opt-in sets leaderboard_opt_in=0,
  hiding the player without deleting their row (scores preserved for re-opt-in)
- SyncProvider trait: opt_out_leaderboard() default no-op method + blanket impl
- SolitaireServerClient: implements opt_out_leaderboard via DELETE request with JWT refresh
- Leaderboard UI: "Opt Out" button (dark red) alongside existing "Opt In" button
- Server integration test: opt-out hides, opt-in restores (round-trip verified)

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

solitaire_data/src/lib.rs

@@ -46,6 +46,11 @@ pub trait SyncProvider: Send + Sync {
     async fn opt_in_leaderboard(&self, _display_name: &str) -> Result<(), SyncError> {
         Ok(())
     }
+    /// Remove the authenticated player from the leaderboard.
+    /// No-op for backends that don't support leaderboards.
+    async fn opt_out_leaderboard(&self) -> Result<(), SyncError> {
+        Ok(())
+    }
 }
 
 /// Blanket impl so `Box<dyn SyncProvider + Send + Sync>` (returned by
@@ -76,6 +81,9 @@ impl SyncProvider for Box<dyn SyncProvider + Send + Sync> {
     async fn opt_in_leaderboard(&self, display_name: &str) -> Result<(), SyncError> {
         (**self).opt_in_leaderboard(display_name).await
     }
+    async fn opt_out_leaderboard(&self) -> Result<(), SyncError> {
+        (**self).opt_out_leaderboard().await
+    }
 }
 
 pub mod stats;