Fix Plex server connection handling and add reachability testing
- Filter relay connections from Plex server discovery (2Mbps bandwidth limit)
- Remove incorrect publicAddressMatches filtering (browser vs server context)
- Add connection reachability testing during login flow
- Fix race condition in Login.tsx causing false 'failed to connect' errors
- Add plex.direct URL validation with user-friendly error message
- Refactor Jellyfin verifyServerAdmin to return structured error codes