feat(data): SyncProvider::delete_account + SolitaireServerClient impl

Adds delete_account() as a default no-op on the SyncProvider trait.
SolitaireServerClient sends DELETE /api/account with JWT (retry on 401).
The server handler already existed (DELETE /api/account, ON DELETE CASCADE).

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

solitaire_data/src/lib.rs

@@ -51,6 +51,11 @@ pub trait SyncProvider: Send + Sync {
     async fn opt_out_leaderboard(&self) -> Result<(), SyncError> {
         Ok(())
     }
+    /// Permanently delete the authenticated player's account and all server
+    /// data. No-op for backends that don't support account management.
+    async fn delete_account(&self) -> Result<(), SyncError> {
+        Ok(())
+    }
 }
 
 /// Blanket impl so `Box<dyn SyncProvider + Send + Sync>` (returned by
@@ -84,6 +89,9 @@ impl SyncProvider for Box<dyn SyncProvider + Send + Sync> {
     async fn opt_out_leaderboard(&self) -> Result<(), SyncError> {
         (**self).opt_out_leaderboard().await
     }
+    async fn delete_account(&self) -> Result<(), SyncError> {
+        (**self).delete_account().await
+    }
 }
 
 pub mod stats;