Upload progress spinner + duplicate game detection

Two features:

1. Progress indicator: when upload starts, the dialog switches from
   IGDB results to a centered spinner with 'Uploading and processing'
   text + filename. The Cancel button stays visible but disabled.

2. Duplicate game blocking: before upload, checks if a game with the
   same title (case-insensitive) already exists in the library.
   Frontend checks against the loaded games list; backend also
   checks as a second line of defense. Shows ⚠️ message in dialog.
   Existing unique-ID suffix logic (foo-2, foo-3) is still in place
   but now only reached when titles actually differ.

src/main/java/com/dostalgia/UploadResource.java

@@ -48,6 +48,15 @@ public class UploadResource {
 
         String gameId = GameService.sanitizeId(title);
 
+        // Check for duplicate by title (case-insensitive)
+        for (var g : svc.list()) {
+            if (g.getTitle() != null && g.getTitle().equalsIgnoreCase(title)) {
+                return Response.status(409)
+                    .entity(Map.of("error", "\"" + title + "\" is already in your library"))
+                    .build();
+            }
+        }
+
         // Ensure unique ID
         String baseId = gameId;
         int counter = 2;