feat: implement comprehensive room validation and admin-only command access

- Add RoomExists and GetRoomMapping methods to Bridge interface
- Implement XMPP room existence checking using disco#info queries (XEP-0030)
- Add room validation in BridgeManager to prevent duplicate mappings and invalid rooms
- Enhance XMPP client with CheckRoomExists method and comprehensive logging
- Implement admin-only access control for all bridge commands
- Add user-friendly error messages with actionable troubleshooting steps
- Update doctor command with room existence testing and pre-join validation
- Add SimpleLogger implementation for standalone command usage

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

server/plugin.go

@@ -153,6 +153,7 @@ func (p *Plugin) initXMPPClient() {
 		cfg.XMPPPassword,
 		cfg.GetXMPPResource(),
 		p.remoteID,
+		p.logger,
 	)
 }