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.

public/css/style.css

@@ -416,11 +416,20 @@ html, body {
   border-radius: 4px;
 }
 
+.offline-data-panel__region-list li {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 6px;
+}
+
 .offline-data-panel__region-list label {
   display: flex;
   align-items: center;
   gap: 6px;
   padding: 4px 8px;
+  min-width: 0;
+  flex: 1;
   cursor: pointer;
 }
 
@@ -428,6 +437,34 @@ html, body {
   background: #f0f0f0;
 }
 
+.offline-data-panel__cache-info {
+  display: flex;
+  align-items: center;
+  gap: 4px;
+  padding-right: 8px;
+  color: #888;
+  font-size: 11px;
+  white-space: nowrap;
+}
+
+.offline-data-panel__cache-delete {
+  border: none;
+  background: none;
+  padding: 0 2px;
+  color: #a33;
+  font-size: 11px;
+  cursor: pointer;
+}
+
+.offline-data-panel__cache-delete:hover {
+  text-decoration: underline;
+}
+
+.offline-data-panel__cache-delete:disabled {
+  opacity: 0.5;
+  cursor: default;
+}
+
 /* Same rounded-square badge ratio as icons.js's own MAP_MARKER_ICON
    (rx 5 of a 22-wide square) — a consistent frame rather than however
    each flag's own artwork happens to fill an unbordered square. */