Corrections to the previous weather-map pass:
* The "helper" that was supposed to live in the top-right was the
color legend, not the layer description. Move the Leaflet legend
control to `topright` (was `bottomright` on desktop).
* Put the layer description back in the desktop sidebar where it
used to live, now with a "Group · Label" header above the body
text so it matches the new style in the mockup.
* Add play/stop controls to the weather forecast timeline, ported
from the propagation map: start plays "now" → end at 1s/step and
loops; stop returns to the time closest to wall clock. A manual
click interrupts playback the same way it does on /map.
* Fix the reconnect/reload loop I introduced by making mount do a
synchronous ProfilesFile read + derive for the full 92k-point
grid. That was pushing mount past the LV socket join timeout on
a cold pod, which makes the LV client fall back to a full HTTP
reload every ~20 seconds (no server-side error). Mount now uses
the original `latest_weather_grid/1` cache-only hot path and only
`weather_grid_at/2` (sync disk read) on user timeline scrubs.
* Clean up the playback timer in the hook's `destroyed` callback.
Graham McIntire
33347f7fc2