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.

app.rb

@@ -291,6 +291,15 @@ module Maptool
       json ok: true
     end
 
+    # What's actually in the per-country download cache right now (not the
+    # same as offline.pmtiles' own meta.sources — see cached_extracts) —
+    # read-only, same tier as GET /api/offline-regions above, so the region
+    # picker can show a size + delete affordance next to each cached
+    # country regardless of whether it's still selected or already built.
+    get "/api/offline-tiles/cache" do
+      json extracts: PmtilesBuilder.cached_extracts(DATA_DIR)
+    end
+
     # Deletes one country's cached Geofabrik extract only — e.g. to force a
     # clean re-download of just that one, or reclaim space for a country
     # that's no longer selected (the cache otherwise keeps it indefinitely).