feat(extension): add popup command bar feedback, sync status, and config commands
Enhances the browser extension popup with:
- Success/error feedback for all commands (green checkmark or red X)
- Sync status bar showing last sync time, pending count, manual sync button
- New commands: server, key, sync for managing sync configuration
Commands:
- tag <tags>: Tag current URL with comma-separated tags
- note <text>: Save text note attached to current page
- search <query>: Search saved items
- server [url]: Show or set sync server URL
- key [apikey]: Show (masked) or set API key
- sync: Trigger sync, shows pull/push counts
UI auto-closes after success (600ms), stays open on error.