feat: Enhance game uniqueness validation to include remote ID and source, and redesign the add game form UI with submission state and feedback.

backend/src/main.rs

@@ -65,7 +65,9 @@ async fn add_game(
     let mut games = game_list.lock().await;
     let mut game = game.into_inner();
 
-    if games.iter().any(|g| g.title == game.title) {
+    if games.iter().any(|g| {
+        g.title == game.title || (g.remote_id == game.remote_id && g.source == game.source)
+    }) {
         return None;
     }