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>

vercel.json

@@ -0,0 +1,8 @@
+{
+  "rewrites": [
+    {
+      "source": "/dropbox-file/:path*",
+      "destination": "https://dl.dropboxusercontent.com/:path*"
+    }
+  ]
+}