Fix rover planner: manual Calculate button, non-blocking computation

- Add explicit "Calculate Coverage" button instead of auto-triggering
- Move coverage computation to background Task so LV stays responsive
  (users can keep adding/removing stations while it computes)
- Remove all auto-compute triggers from station add/remove/band change
- URL loading resolves stations but doesn't auto-calculate
- Button shows "Finding best roving locations..." spinner while computing

lib/microwaveprop_web/live/rover_live.ex

@@ -80,17 +80,24 @@ defmodule MicrowavepropWeb.RoverLive do
     idx = String.to_integer(idx_str)
     stations = List.delete_at(socket.assigns.stations, idx)
     socket = assign(socket, stations: stations, coverage: [], selected_grid: nil)
-    if length(stations) >= 2, do: send(self(), :compute_coverage)
     socket = push_event(socket, "stations_updated", %{stations: station_data(stations)})
     {:noreply, push_url(socket)}
   end
 
   def handle_event("select_band", %{"band" => band_str}, socket) do
     socket = assign(socket, band: String.to_integer(band_str), coverage: [], selected_grid: nil)
-    if length(socket.assigns.stations) >= 2, do: send(self(), :compute_coverage)
     {:noreply, push_url(socket)}
   end
 
+  def handle_event("calculate", _params, socket) do
+    if length(socket.assigns.stations) >= 2 and not socket.assigns.computing do
+      send(self(), :compute_coverage)
+      {:noreply, assign(socket, computing: true)}
+    else
+      {:noreply, socket}
+    end
+  end
+
   def handle_event("select_grid", %{"grid" => grid}, socket) do
     detail = Enum.find(socket.assigns.coverage, &(&1.grid == grid))
     {:noreply, assign(socket, selected_grid: detail)}
@@ -112,7 +119,6 @@ defmodule MicrowavepropWeb.RoverLive do
       |> assign(stations: stations, resolving: false)
       |> push_event("stations_updated", %{stations: station_data(stations)})
 
-    if length(stations) >= 2, do: send(self(), :compute_coverage)
     {:noreply, socket}
   end
 
@@ -123,7 +129,6 @@ defmodule MicrowavepropWeb.RoverLive do
         stations = socket.assigns.stations ++ [station]
         socket = assign(socket, stations: stations, resolving: false)
         socket = push_event(socket, "stations_updated", %{stations: station_data(stations)})
-        if length(stations) >= 2, do: send(self(), :compute_coverage)
         {:noreply, push_url(socket)}
 
       {:error, _reason} ->
@@ -134,11 +139,52 @@ defmodule MicrowavepropWeb.RoverLive do
   def handle_info(:compute_coverage, socket) do
     stations = socket.assigns.stations
     band_mhz = socket.assigns.band
+
+    if length(stations) < 2 do
+      {:noreply, socket}
+    else
+      # Run in a background task so the LV stays responsive
+      pid = self()
+
+      Task.start(fn ->
+        result = do_compute_coverage(stations, band_mhz)
+        send(pid, {:coverage_result, result})
+      end)
+
+      {:noreply, assign(socket, computing: true)}
+    end
+  end
+
+  def handle_info({:coverage_result, coverage}, socket) do
+    stations = socket.assigns.stations
+
+    grid_data =
+      Enum.map(coverage, fn c ->
+        %{
+          grid: c.grid,
+          lat: c.lat,
+          lon: c.lon,
+          coverage_score: c.coverage_score,
+          stations_in_range: c.stations_in_range,
+          workable_count: c.workable_count,
+          total_stations: length(stations),
+          prop_score: c.prop_score,
+          best_hour: c.best_hour
+        }
+      end)
+
+    socket =
+      socket
+      |> assign(coverage: coverage, computing: false, selected_grid: nil)
+      |> push_event("coverage_updated", %{grids: grid_data})
+
+    {:noreply, socket}
+  end
+
+  defp do_compute_coverage(stations, band_mhz) do
     band_config = BandConfig.get(band_mhz) || BandConfig.get(10_000)
     max_range = band_config.extended_range_km || 500
 
-    socket = assign(socket, computing: true)
-
     # Find the bounding box around all stations, expanded by max range
     lats = Enum.map(stations, & &1.lat)
     lons = Enum.map(stations, & &1.lon)
@@ -180,28 +226,7 @@ defmodule MicrowavepropWeb.RoverLive do
       end)
       |> Enum.sort_by(& &1.coverage_score, :desc)
 
-    # Push to JS for rendering
-    grid_data =
-      Enum.map(coverage, fn c ->
-        %{
-          grid: c.grid,
-          lat: c.lat,
-          lon: c.lon,
-          coverage_score: c.coverage_score,
-          stations_in_range: c.stations_in_range,
-          workable_count: c.workable_count,
-          total_stations: length(stations),
-          prop_score: c.prop_score,
-          best_hour: c.best_hour
-        }
-      end)
-
-    socket =
-      socket
-      |> assign(coverage: coverage, computing: false, selected_grid: nil)
-      |> push_event("coverage_updated", %{grids: grid_data})
-
-    {:noreply, socket}
+    coverage
   end
 
   # ── Coverage Scoring ──
@@ -433,16 +458,25 @@ defmodule MicrowavepropWeb.RoverLive do
           </div>
         <% end %>
 
-        <%= if @computing do %>
-          <div class="text-xs text-center py-3">
-            <span class="loading loading-spinner loading-sm"></span>
-            <div class="mt-1">Computing coverage areas...</div>
-          </div>
+        <%= if length(@stations) >= 2 do %>
+          <button
+            type="button"
+            phx-click="calculate"
+            class="btn btn-primary btn-sm w-full"
+            disabled={@computing}
+          >
+            <%= if @computing do %>
+              <span class="loading loading-spinner loading-sm"></span>
+              Finding best roving locations...
+            <% else %>
+              Calculate Coverage
+            <% end %>
+          </button>
         <% end %>
 
-        <%= if @stations != [] and !@computing and @coverage == [] do %>
+        <%= if length(@stations) == 1 do %>
           <div class="text-xs opacity-50 text-center py-2">
-            Add at least 2 stations to see coverage
+            Add at least 2 stations to calculate
           </div>
         <% end %>