Extend grid validation and decoding from 4/6 chars to unlimited
even-length grids (8, 10, etc). Recursive decoder handles alternating
letter(/24) and digit(/10) subdivision pairs after the field+square.
Validated against gridmap-web's Maidenhead implementation.
Maidenhead grid module converts grid squares to lat/lon coordinates.
Submission form validates grids, bands, modes, and email, computes
positions and distance, then triggers the weather/HRRR/terrain
processing pipeline via Oban.
