Add grid toggle, use 0.25deg resolution for coverage scoring

- Toggle button for Maidenhead grid overlay (on by default)
- Coverage candidates now at 0.25° resolution (~28 km) instead of
  2°×1° Maidenhead grids, matching HRRR propagation grid density
- Coverage rectangles render as 0.25° cells for finer detail
- Grid overlay is separate from coverage coloring

assets/js/rover_map_hook.js

@@ -19,6 +19,9 @@ export const RoverMap = {
     this.stationMarkers.addTo(map)
     this.coverageLayer.addTo(map)
 
+    this.gridLayer = L.layerGroup().addTo(map)
+    this.drawGridOverlay()
+
     this.handleEvent("stations_updated", ({ stations }) => {
       this.stations = stations
       this.renderStations()
@@ -27,6 +30,14 @@ export const RoverMap = {
     this.handleEvent("coverage_updated", ({ grids }) => {
       this.renderCoverage(grids)
     })
+
+    this.handleEvent("toggle_grids", ({ show }) => {
+      if (show) {
+        this.gridLayer.addTo(this.map)
+      } else {
+        this.map.removeLayer(this.gridLayer)
+      }
+    })
   },
 
   renderStations() {
@@ -64,10 +75,9 @@ export const RoverMap = {
     const maxScore = Math.max(...grids.map(g => g.coverage_score))
 
     for (const g of grids) {
-      // 4-char Maidenhead: 2° lon × 1° lat
-      // Compute grid square bounds from the grid letters
-      const bounds = this.gridBounds(g.grid)
-      if (!bounds) continue
+      // Render as 0.25° cells centered on the candidate point
+      const half = 0.125
+      const bounds = [[g.lat - half, g.lon - half], [g.lat + half, g.lon + half]]
 
       const normalized = maxScore > 0 ? g.coverage_score / maxScore : 0
       const color = this.coverageColor(normalized)
@@ -149,6 +159,19 @@ export const RoverMap = {
     return "#ff4f4f"
   },
 
+  drawGridOverlay() {
+    for (let lat = 25; lat <= 50; lat += 1) {
+      L.polyline([[lat, -125], [lat, -66]], {
+        color: "#888", weight: 0.5, opacity: 0.3
+      }).addTo(this.gridLayer)
+    }
+    for (let lon = -125; lon <= -66; lon += 2) {
+      L.polyline([[25, lon], [50, lon]], {
+        color: "#888", weight: 0.5, opacity: 0.3
+      }).addTo(this.gridLayer)
+    }
+  },
+
   fitBounds() {
     const points = this.stations.map(s => [s.lat, s.lon])
     if (points.length >= 2) {

lib/microwaveprop/rover/coverage.ex

@@ -79,20 +79,21 @@ defmodule Microwaveprop.Rover.Coverage do
     min_lon = max(Enum.min(lons) - range_deg, -125.0)
     max_lon = min(Enum.max(lons) + range_deg, -66.0)
 
-    # Generate at 4-char Maidenhead resolution (1° lat × 2° lon)
-    lat_range = Enum.to_list(trunc(min_lat)..trunc(max_lat))
-    lon_range = Enum.to_list(trunc(min_lon)..trunc(max_lon)//2)
+    # Generate candidates at 0.25° resolution (between HRRR 0.125° and Maidenhead 1-2°)
+    # This gives ~16x more candidates than Maidenhead but stays manageable
+    step = 0.25
 
-    # Use center of grid square
-    for_result =
-      for lat <- lat_range, lon <- lon_range do
-        clat = lat + 0.5
-        clon = lon + 1.0
-        grid = Geo.latlon_to_grid4(clat, clon)
-        {grid, clat, clon}
-      end
+    for lat <- float_range(min_lat, max_lat, step),
+        lon <- float_range(min_lon, max_lon, step) do
+      grid = Geo.latlon_to_grid4(lat, lon)
+      {grid, Float.round(lat, 3), Float.round(lon, 3)}
+    end
+  end
 
-    Enum.uniq_by(for_result, fn {grid, _, _} -> grid end)
+  defp float_range(min, max, step) do
+    (Float.ceil(min / step) * step)
+    |> Stream.iterate(&(&1 + step))
+    |> Enum.take_while(&(&1 <= max))
   end
 
   defp fast_score_grid(grid, lat, lon, stations, band_mhz, max_range) do

lib/microwaveprop_web/live/rover_live.ex

@@ -32,7 +32,8 @@ defmodule MicrowavepropWeb.RoverLive do
        selected_grid: nil,
        resolving: false,
        computing: false,
-       url_loaded: false
+       url_loaded: false,
+       show_grids: true
      )}
   end
 
@@ -80,6 +81,11 @@ defmodule MicrowavepropWeb.RoverLive do
     {:noreply, push_url(socket)}
   end
 
+  def handle_event("toggle_grids", _params, socket) do
+    show = !socket.assigns.show_grids
+    {:noreply, socket |> assign(show_grids: show) |> push_event("toggle_grids", %{show: show})}
+  end
+
   def handle_event("select_band", %{"band" => band_str}, socket) do
     socket = assign(socket, band: String.to_integer(band_str), coverage: [], selected_grid: nil)
     {:noreply, push_url(socket)}
@@ -254,11 +260,21 @@ defmodule MicrowavepropWeb.RoverLive do
       <div class="absolute top-2 left-2 z-[1000] w-80 max-h-[calc(100vh-1rem)] overflow-y-auto bg-base-100/95 shadow-lg rounded-box border border-base-300 p-3 flex flex-col gap-3">
         <div class="font-bold text-sm">Rover Planner</div>
 
-        <select phx-change="select_band" name="band" class="select select-bordered select-sm w-full">
-          <%= for {label, value} <- @band_options do %>
-            <option value={value} selected={value == to_string(@band)}>{label}</option>
-          <% end %>
-        </select>
+        <div class="flex gap-2">
+          <select phx-change="select_band" name="band" class="select select-bordered select-sm flex-1">
+            <%= for {label, value} <- @band_options do %>
+              <option value={value} selected={value == to_string(@band)}>{label}</option>
+            <% end %>
+          </select>
+          <button
+            type="button"
+            phx-click="toggle_grids"
+            class={["btn btn-sm btn-square", if(@show_grids, do: "btn-primary", else: "btn-ghost")]}
+            title="Toggle grid overlay"
+          >
+            <.icon name="hero-squares-2x2" class="size-4" />
+          </button>
+        </div>
 
         <form phx-submit="add_station" class="flex gap-1">
           <input