feat(rf): RF Environment tab exposing spectral scan + noise floor

[iceice400]

Summary

• Add GET /api/rf/status exposing NoiseFloorTracker snapshot and spectral-scan service state (enabled/supported/running, scan counters, latest histogram).
• Retain the most recent scan histogram on SpectralScanService for the RF dashboard (no new HAL code).
• Add RF Environment sidebar tab with noise-floor sparkline, live vs packet-fallback badges, calibration state, and Chart.js spectral histogram.

Test plan

• python -m unittest tests.test_rf_routes tests.test_spectral_scan_service -v (7 passed)
• Open RF Environment tab — noise floor readout and sparkline populate
• With spectral_scan_interval_seconds: 0 — tab shows disabled scan message, packet fallback badge
• On hardware with scan enabled — histogram renders after first scan; badge shows Live scan
• Sidebar telemetry rail still works independently

AI disclosure

Implemented with Cursor AI assistance; reviewed and tested locally.