Add vercel.json for zero-config deploys of the demo site

Builds core -> widget -> demo and serves packages/demo/dist as the output
directory, so importing the repo in Vercel needs no manual build settings.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01GWJbXXhvTx9wo59jSz57J6

vercel.json

@@ -0,0 +1,5 @@
+{
+  "buildCommand": "npm run build -w @bitlogin/core && npm run build -w @bitlogin/widget && npm run build -w @bitlogin/demo",
+  "outputDirectory": "packages/demo/dist",
+  "cleanUrls": true
+}