- Toggle button for Maidenhead grid overlay (on by default)
- Coverage candidates now at 0.25° resolution (~28 km) instead of
2°×1° Maidenhead grids, matching HRRR propagation grid density
- Coverage rectangles render as 0.25° cells for finer detail
- Grid overlay is separate from coverage coloring
- Extract scoring logic to Microwaveprop.Rover.Coverage for testability
- Two-phase computation: fast pass (distance + propagation) for all grids,
then SRTM terrain analysis for top 20 candidates only
- Cap search radius to 300 km to keep candidate count reasonable
- Run terrain analysis in Task with rescue/fallback for resilience
- Background Task doesn't block LiveView process
- 5 tests covering: ranked results, empty inputs, station details,
band range differences, field presence
