fix(fetch): use Vite proxy + Vercel rewrites for reliable Dropbox loading

Replace unreliable third-party CORS proxies (allorigins, cors-anywhere,
thingproxy) with a local /dropbox-file route proxied server-side:
- vite.config.ts: add dev server proxy for /dropbox-file -> dl.dropboxusercontent.com
- vercel.json: add rewrite rule for the same route in production
- App.tsx: simplify fetch logic to use single reliable proxy path

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

vite.config.ts

@@ -17,8 +17,15 @@ export default defineConfig(({mode}) => {
     },
     server: {
       // HMR is disabled in AI Studio via DISABLE_HMR env var.
-      // Do not modify—file watching is disabled to prevent flickering during agent edits.
+      // Do not modify - file watching is disabled to prevent flickering during agent edits.
       hmr: process.env.DISABLE_HMR !== 'true',
+      proxy: {
+        '/dropbox-file': {
+          target: 'https://dl.dropboxusercontent.com',
+          changeOrigin: true,
+          rewrite: (path) => path.replace(/^\/dropbox-file/, ''),
+        },
+      },
     },
   };
 });