Upload: clearer proxy 413 error, clean up after interrupted uploads

- Show a readable message when a reverse proxy rejects the archive with 413
- Delete the freshly created game if its archive upload fails
- Turn ClientDisconnect during upload into a short 400 instead of a traceback

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/pages/NewGame.tsx

@@ -16,8 +16,9 @@ export function NewGame() {
     if (!file) return;
     setError("");
     setProgress(0);
+    let game: Game | null = null;
     try {
-      const game = await api.post<Game>("/api/games", {
+      game = await api.post<Game>("/api/games", {
         title: title.trim() || file.name.replace(/\.(zip|7z|rar|tar.*)$/i, ""),
         description,
         tags: tags.split(",").map((t) => t.trim()).filter(Boolean),
@@ -26,6 +27,8 @@ export function NewGame() {
                       file, setProgress);
       navigate(`/games/${game.id}`);
     } catch (err) {
+      // don't leave an empty game card behind when the archive never arrived
+      if (game) await api.del(`/api/games/${game.id}`).catch(() => {});
       setError((err as Error).message);
       setProgress(null);
     }