Analytics: add an RF and Signal page with distributions and trends #54

Add an RF/Signal analytics page with colorful SNR and RSSI distributions, hourly reception-quality trends, sample coverage, and exact accessible counts.

Use the new bounded server signal aggregate contract, with 24h/7d/30d and the global region selector. Show units and actual sample counts; preserve missing hours/measurements as gaps. Reception signal describes the last hop reported by observers, not end-to-end link quality or packet loss. Invalid/unavailable readings must not appear as healthy zero values.

Reuse the existing ECharts theme, chart cards and query cancellation/filter patterns. Handle loading, empty, partial and failed data; verify mobile/desktop layouts and single-hour windows. No chart dependency or synthetic production data. Keep this as a small dependent PR after #53, with a focused feature comparison and native Pi preview/changelog validation.