chore(vercel): disable auto-deploys for the unstable branch

Pushing to unstable triggered a Vercel Preview build on every commit (and the
manual promote rebuilt again) — wasted build time on a fast-moving dev branch we
deploy on demand anyway. Set git.deploymentEnabled.unstable=false so pushes no
longer auto-build; deploys are now explicit via `vercel deploy --prod` when we
want unstable.bitvid.network updated. beta/main keep auto-deploying (intentional,
low-frequency promotions).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

vercel.json

@@ -2,6 +2,11 @@
   "buildCommand": "npm run build",
   "outputDirectory": "dist",
   "installCommand": "npm install",
+  "git": {
+    "deploymentEnabled": {
+      "unstable": false
+    }
+  },
   "headers": [
     {
       "source": "/sw.min.js",