Show what's actually cached per-country, with a delete button per row

The only visibility into downloaded data was the whole offline.pmtiles
row, and the only delete was "everything" (whole file + whole cache).
GET /api/offline-tiles/cache lists what's really in
data/geofabrik_cache/ right now — distinct from offline.pmtiles' own
meta.sources, which only reflects the last successful build and can
disagree with both the current selection and the cache (ticked-but-
not-Applied, or built-from-but-since-unticked).

The region picker now shows each cached country's size next to its
checkbox, with its own delete button (DELETE
/api/offline-tiles/cache/:code, already added but unused by the UI
until now) — removing just that entry rather than clearing the whole
cache.

test/test_pmtiles_builder.rb

@@ -338,6 +338,23 @@ class PmtilesBuilderTest < Minitest::Test
     assert_nil Maptool::PmtilesBuilder.terrain_meta(@data_dir)
   end
 
+  def test_cached_extracts_is_empty_before_any_download
+    assert_empty Maptool::PmtilesBuilder.cached_extracts(@data_dir)
+  end
+
+  # Deliberately still selected — cached_extracts reflects the cache on
+  # disk, not the current selection or the last build's own meta (see
+  # cached_extracts' own comment for why those three can all disagree).
+  def test_cached_extracts_lists_what_was_downloaded
+    select_regions("SK", "AT")
+    Maptool::PmtilesBuilder.build_offline(data_dir: @data_dir, download_extract: fake_download).join
+
+    extracts = Maptool::PmtilesBuilder.cached_extracts(@data_dir)
+    assert_equal %w[AT SK], extracts.map { |e| e["code"] }
+    assert_equal [4, 4], extracts.map { |e| e["size"] } # "fake".bytesize
+    assert_equal ["Wed, 10 Sep 2026 08:00:01 GMT"] * 2, extracts.map { |e| e["last_modified"] }
+  end
+
   def test_offline_meta_is_nil_before_any_build
     assert_nil Maptool::PmtilesBuilder.offline_meta(@data_dir)
   end