Give Overpass POI downloads icons/descriptions and unique filenames

Overpass categories now carry the same icon+color styling a p4n2gpx
waypoint gets, instead of falling back to gpx.js's plain red marker,
plus a popup description built from opening_hours/operator/website
tags. Supermarket/convenience store reuse NPS's generic store glyph,
the only honest match in that icon set.

Each plugin run's saved filename now bakes in the area's center and a
timestamp, so repeated downloads for different areas land as separate,
identifiable files instead of overpass-pois.geojson/-1/-2/...

test/test_overpass_pois.rb

@@ -22,6 +22,44 @@ class OverpassPoisTest < Minitest::Test
     water = geojson["features"].find { |f| f["properties"]["category"] == "Drinking water" }
     assert_equal [17.2, 48.1], water["geometry"]["coordinates"]
     assert_equal "Drinking water", water["properties"]["name"], "falls back to the category when the node has no name tag"
+    assert_equal "amenity_drinking_water", water["properties"]["icon"]
+    assert_equal "#2e86de", water["properties"]["color"]
+  end
+
+  def test_to_geojson_assigns_icons_to_shop_categories
+    response = {
+      "elements" => [
+        { "type" => "node", "lat" => 1, "lon" => 1, "tags" => { "shop" => "supermarket" } },
+        { "type" => "node", "lat" => 2, "lon" => 2, "tags" => { "shop" => "convenience" } },
+      ],
+    }
+
+    icons = Maptool::OverpassPois.to_geojson(response)["features"].map { |f| f["properties"]["icon"] }
+
+    assert_equal %w[shop_supermarket shop_convenience], icons
+  end
+
+  def test_to_geojson_builds_a_description_from_recognized_tags
+    response = {
+      "elements" => [
+        {
+          "type" => "node",
+          "lat" => 48.1,
+          "lon" => 17.2,
+          "tags" => { "amenity" => "toilets", "opening_hours" => "24/7", "operator" => "Town Council", "website" => "https://example.com" },
+        },
+      ],
+    }
+
+    description = Maptool::OverpassPois.to_geojson(response)["features"].first["properties"]["description"]
+
+    assert_equal "Opening hours: 24/7\nOperator: Town Council\nhttps://example.com", description
+  end
+
+  def test_to_geojson_omits_description_when_no_recognized_tags_are_present
+    response = { "elements" => [{ "type" => "node", "lat" => 48.1, "lon" => 17.2, "tags" => { "amenity" => "toilets" } }] }
+
+    refute Maptool::OverpassPois.to_geojson(response)["features"].first["properties"].key?("description")
   end
 
   def test_to_geojson_uses_the_node_name_when_present