Extract shared Maidenhead grid overlay into maidenhead_grid.js so both
the main map and rover planner use the same implementation. Wire up the
toggle_grid event and checkbox on the rover page matching the main map.
Extract GridSquare class and updateGridOverlay from propagation_map_hook
into shared maidenhead_grid.js module. Rover map now shows the same
zoom-adaptive Maidenhead grid overlay as the main /map page — red
rectangles with labels that switch between field (2-char) and square
(4-char) based on zoom level. Updates on pan/zoom.
Toggle button hides/shows the grid overlay.
