Fix App.js: guard list.find against non-array response

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/App.js

@@ -188,7 +188,7 @@ function App() {
     async function pickBackground() {
       try {
         const res = await axios.get('/api/gmkit/list');
-        const list = res.data || [];
+        const list = Array.isArray(res.data) ? res.data : [];
         if (!mounted) return;
         // prefer explicit deathwatch-banner.png if present
         const banner = list.find(f => f.name === 'deathwatch-banner.png');