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.

lib/maptool/pmtiles_builder.rb

@@ -448,6 +448,26 @@ module Maptool
       File.delete("#{path}.last_modified") if File.exist?("#{path}.last_modified")
     end
 
+    # What's actually sitting in the per-country download cache right now —
+    # not the same thing as offline_meta's "sources", which only reflects
+    # whatever the *last successful build* used. A country can be cached
+    # without ever having been built (selected, then deselected again
+    # before Apply) or built from without still being selected, and this is
+    # what lets the region picker show/clear either case.
+    def self.cached_extracts(data_dir)
+      cache_dir = geofabrik_cache_dir(data_dir)
+      return [] unless Dir.exist?(cache_dir)
+
+      Dir.glob(File.join(cache_dir, "*.osm.pbf")).map do |path|
+        stamp_path = "#{path}.last_modified"
+        {
+          "code" => File.basename(path, ".osm.pbf"),
+          "size" => File.size(path),
+          "last_modified" => File.exist?(stamp_path) ? File.read(stamp_path) : nil,
+        }
+      end.sort_by { |s| s["code"] }
+    end
+
     # --- terrain.pmtiles ---
 
     # planet.pmtiles (z0-12, always) plus whichever regional chunks