fix(weather): drop expensive grid fetch from mount to avoid LV join timeout

/weather mount called Weather.latest_weather_grid/1 directly, which
runs WeatherLayers.derive over 92k cells and Jason-encodes the
result. Production HTTP logs showed dead renders consistently
landing at 9.8–10.2 s — right at LiveView's socket join deadline.
The browser would render half the page, the WS would fail to mount,
and the user saw "attempting to reconnect".

The JS hook already fires map_bounds via requestAnimationFrame
immediately after mount, and the existing handle_event("map_bounds")
path returns viewport-scoped weather data via the update_weather
push event. So the dead-render data fetch is redundant — drop it.
Mount now returns initial_data_json: "[]"; the layer paints
~50–200 ms after WS connect via the bounds-driven flow.

lib/microwaveprop_web/live/weather_map_live.ex

@@ -173,27 +173,22 @@ defmodule MicrowavepropWeb.WeatherMapLive do
     # the underlying cache can advance to while the worker is walking
     # f00→f18).
     #
-    # Hot-path mount: when the closest-to-now time happens to be the
-    # cached latest, use `latest_weather_grid/1` so mount stays under
-    # the LV socket join timeout (~10s). A ProfilesFile read for any
-    # other hour is a single ~2 MB ETF decode, also fine for mount.
+    # Mount stays cheap — no grid materialisation. The dead render
+    # would otherwise spend ~5–10 s materialising 92k cells through
+    # `WeatherLayers.derive`, blocking the HTTP response and tripping
+    # the LiveView socket join timeout. The JS hook fires `map_bounds`
+    # via `requestAnimationFrame` immediately after mount, and the
+    # existing `handle_event("map_bounds", ...)` path drives the
+    # initial layer paint via `update_weather`.
     valid_times = Weather.available_weather_valid_times()
-    latest_vt = Weather.latest_grid_valid_time()
     initial_vt = pick_initial_valid_time(valid_times)
 
-    data =
-      cond do
-        is_nil(initial_vt) -> []
-        initial_vt == latest_vt -> Weather.latest_weather_grid(@initial_bounds)
-        true -> Weather.weather_grid_at(initial_vt, @initial_bounds)
-      end
-
     {:ok,
      assign(socket,
        page_title: "Weather Map",
        layers: @layers,
        selected_layer: "temperature",
-       initial_data_json: Jason.encode!(data),
+       initial_data_json: "[]",
        valid_time: initial_vt,
        valid_times: valid_times,
        initial_valid_times_json: Jason.encode!(Enum.map(valid_times, &DateTime.to_iso8601/1)),