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_app.rb

@@ -307,7 +307,7 @@ class AppTest < Minitest::Test
     )
     assert_equal 200, response.status
     body = json_body(response)
-    assert_equal "test-plugin.geojson", body["path"]
+    assert_match(/\Atest-plugin_48\.05_17\.05_\d{6}_\d{4}\.geojson\z/, body["path"])
 
     saved = json_body(request.get("/api/gpx/file/#{body["path"]}"))
     assert_equal "Test POI", saved["features"][0]["properties"]["name"]