feat(cli): add remote proxy support with token auth and optional --port

- Add --token option (also reads FPB_TOKEN env) for remote server auth
- Add ProxyAuthError with clear guidance when 401/403 is received
- Send X-Auth-Token header on all proxy requests (GET/POST/upload)
- Remote mode: pure proxy, no auto-launch, no direct serial fallback
- Make --port optional in proxy mode (server owns the serial port)
- Replace misleading '(requires --port)' with '(requires device)' in help
- Simplify CLI examples to clarify offline vs device vs remote usage
- Fix pre-existing F541 lint warning in test_auto_ban.py

Author: FASTSHIFT