Short-circuit RangeEstimate below 5.76 GHz to stop browser lockup

For a 1 W / 432 MHz beacon, RangeEstimate.estimate/1 was returning
~126 k cells because free-space loss dominates over atmospheric
loss at low UHF, letting the rx_dbm filter pass almost the whole
bbox. Jason-encoding that payload into the map's data-cells
attribute was locking up browsers on the beacon detail page for
low-band beacons even though the coverage toggle is already
disabled for them. Return an empty estimate below the same 5760
MHz threshold the UI already enforces.

lib/microwaveprop/beacons/range_estimate.ex

@@ -35,6 +35,15 @@ defmodule Microwaveprop.Beacons.RangeEstimate do
   # HRRR grid step (degrees).
   @grid_step 0.125
 
+  # Below this frequency free-space loss dominates over atmospheric
+  # loss, so `rx_dbm` exceeds the detection floor across tens of
+  # thousands of km². On a 1 W / 432 MHz beacon the estimate produced
+  # ~126k cells — Jason-encoding that into the map's data attribute
+  # locked up browsers in prod. Return an empty estimate below the
+  # threshold so the coverage toggle (also gated at 5760 MHz in the
+  # LiveView) never has a payload to render.
+  @min_supported_mhz 5760
+
   @doc """
   Convert a power in milliwatts to dBm. Returns `-999.9` for non-positive input.
   """
@@ -60,6 +69,10 @@ defmodule Microwaveprop.Beacons.RangeEstimate do
   detection floor.
   """
   @spec estimate(struct()) :: map()
+  def estimate(%{frequency_mhz: freq_mhz} = beacon) when freq_mhz < @min_supported_mhz do
+    empty_estimate(beacon)
+  end
+
   def estimate(beacon) do
     band_mhz = nearest_band_mhz(beacon.frequency_mhz)
     band_config = BandConfig.get(band_mhz)
@@ -123,6 +136,25 @@ defmodule Microwaveprop.Beacons.RangeEstimate do
     }
   end
 
+  defp empty_estimate(beacon) do
+    band_mhz = nearest_band_mhz(beacon.frequency_mhz)
+    band_config = BandConfig.get(band_mhz)
+
+    %{
+      beacon_id: beacon.id,
+      band_mhz: band_mhz,
+      band_label: band_config && band_config.label,
+      center_score: 50,
+      valid_time: nil,
+      eirp_dbm: Float.round(mw_to_dbm(beacon.power_mw || 0.0), 1),
+      atm_per_km: 0.0,
+      grid_step: @grid_step,
+      max_range_km: 0.0,
+      cells: [],
+      tiers: @tiers
+    }
+  end
+
   # --- path loss ------------------------------------------------------------
 
   defp received_dbm(_eirp, _f, _atm, +0.0, _score), do: 999.0

test/microwaveprop/beacons/range_estimate_test.exs

@@ -112,5 +112,23 @@ defmodule Microwaveprop.Beacons.RangeEstimateTest do
         assert lon_mod < 0.01
       end
     end
+
+    test "short-circuits with empty cells for beacons below 5.76 GHz" do
+      # 432 MHz with 1 W at unrestricted range would generate >100k cells
+      # because atmospheric loss is negligible — encoding that payload
+      # into the map's data attribute was locking up browsers in prod.
+      result = RangeEstimate.estimate(beacon(frequency_mhz: 432.38, power_mw: 1000.0))
+      assert result.cells == []
+      assert result.max_range_km == 0.0
+      # Still reports the nearest configured band for display purposes.
+      assert is_integer(result.band_mhz)
+    end
+
+    test "short-circuits at exactly 5760 MHz threshold" do
+      above = RangeEstimate.estimate(beacon(frequency_mhz: 5760.0, power_mw: 1000.0))
+      below = RangeEstimate.estimate(beacon(frequency_mhz: 5759.9, power_mw: 1000.0))
+      assert above.cells != []
+      assert below.cells == []
+    end
   end
 end