Owner / admin click "Edit" on /rover-locations/:id and the marker
becomes draggable. As they drag, the JS hook pushes `location_dragged`
events with the new lat/lon; the LiveView updates `working_lat` /
`working_lon` / `grid` so the page shows the live preview without
writing to the DB. "Save" persists via Rover.update_location, "Cancel"
reverts both the assigns and the marker position.
Implementation notes:
- Switched the map marker from L.circleMarker to a draggable L.marker
with a divIcon (CircleMarker has no `dragging` handler).
- A second `draggingDotIcon` (amber, larger, grab cursor) makes the
edit affordance obvious.
- Server <-> hook coordination uses push_event:
set_marker_draggable / reset_marker. Drag results come back on
pushEvent("location_dragged", { lat, lon }).
- Coordinates row binds to working_lat/working_lon so the displayed
decimals + derived grid update on every drag, not only on save.
LocationMap hook now adds a togglable grid layer (matching the contact
map pattern) so the rover-location detail page shows the standard
Maidenhead overlay alongside the marker.
Click a row's info area to expand a Leaflet mini-map centered on that
pin, defaulting to ESRI World_Imagery satellite (zoom up to 21) with
OSM/Topo toggles in a top-right layer control.
