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.
Two related UI changes driven by the same data:
* Move the selected-layer description out of the sidebar and into a
dedicated top-right overlay on the map. The sidebar kept the same
layer buttons but dropped the descriptive text; the new overlay
shows group + layer name + description in one card. Mobile still
shows the description in the collapsible panel since it has no
top-right free real estate.
* Add a forecast-hour timeline bar at the bottom of the map, matching
the propagation map. WeatherMapLive enumerates ProfilesFile on mount
(the grid worker has been persisting f00..f18 on disk since
commit 07ffcf5), pushes data-valid-times for the JS hook to render
as buttons, and handles a new select_time event by reading the
per-hour ProfilesFile on demand via Weather.weather_grid_at/2.
weather_grid_at/2 deliberately skips the GridCache write path for
non-latest hours — caching 18 × 92k rows would add ~300 MB per pod.
A ~2 MB ETF decode per scrub is fast enough for a click.
Subscribes to propagation:pipeline so the timeline picks up new
forecast hours live as they land, without waiting for the full chain
to finish.
