Display LTE signal measurements (RSRP, RSRQ, RSSI, PCI, EARFCN) from
DIAG ML1 Serving Cell Measurement messages in the web UI.
- Add CellInfo struct with RwLock cache in gsmtap_parser
- Add CellSignalInfo to SystemStats API response
- Add Cell Signal row to SystemStatsTable with quality indicator
- Support Orbic, Tplink, Tmobile, Wingtech devices (graceful degradation for others)