Add version proxy to allow custom notifications

[NJannasch]

Summary

• Route version checks through Cloudflare Worker proxy instead of hitting GitHub API directly
• Support broadcast messages (dismissible + force) shown as banners in the UI
• Dismissible messages persist dismiss state in localStorage; force messages cannot be dismissed

Test plan

• Verify version check still works via /api/version
• Set a broadcast message in admin UI, confirm it appears as a banner
• Dismiss a message, reload — confirm it stays dismissed
• Set a force message, confirm dismiss button is hidden and banner uses red styling
• Clear force message in admin, confirm banner disappears