base-station: add heatmap and coverage-rings display modes

The first mobile-data overlay drew each site as a flat dot, which
hides both the cell's range and the local site density, and every
panel switch refetched the same bbox from the rate-limited
OpenCellID and Overpass endpoints.

- Render two new display modes: `Heatmap` (heat layer keyed on cell
  density) and `CoverageRings` (stacked ring polygons sized by the
  per-tower range estimate).
- Cache OpenCellID and Overpass responses per bbox in the store,
  with LRU trimming to `MOBILE_COVERAGE_CACHE_MAX_ENTRIES`.
- Add a drag-to-fetch target tool for areas outside the default
  auto-fetch radius, plus ring rim labels and a heatmap intensity
  slider in the config panel.
- Default-merge the persisted `mobileCoverageCache` on load so a
  schema bump doesn't crash callers that read its fields.
- Surface an info snackbar on Lite when no API key is set instead
  of failing silently.

Author: ArturoManzoli