refactor(sync): remove mirror_achievement from SyncProvider trait

The method had a no-op default, was never overridden in
SolitaireServerClient, and was never called by any engine system.
Achievements are already synced via the full SyncPayload push, so
the method provided no additional value and was a dead maintenance trap.

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

solitaire_data/src/lib.rs

@@ -27,10 +27,6 @@ pub trait SyncProvider: Send + Sync {
     fn backend_name(&self) -> &'static str;
     /// Returns true if the user is currently authenticated with this backend.
     fn is_authenticated(&self) -> bool;
-    /// Mirror an achievement unlock to this backend (no-op for most backends).
-    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> {
@@ -83,9 +79,6 @@ impl SyncProvider for Box<dyn SyncProvider + Send + Sync> {
     fn is_authenticated(&self) -> bool {
         (**self).is_authenticated()
     }
-    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
     }