build(vercel): pin npm install command (avoid pnpm auto-detect)

Both package-lock.json and pnpm-lock.yaml are present, so Vercel auto-selected
pnpm, whose install failed on the new bitvid-unstable/bitvid-beta projects (the
existing bitvid project only worked via a dashboard install-command override).
Pinning installCommand makes every project/branch build with npm regardless of
dashboard settings.

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

vercel.json

@@ -1,4 +1,5 @@
 {
   "buildCommand": "npm run build",
-  "outputDirectory": "dist"
+  "outputDirectory": "dist",
+  "installCommand": "npm install"
 }