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.

docs/development.adoc

@@ -147,7 +147,15 @@ which only changes what a *future* build covers and never frees
 anything already on disk. `DELETE
 /api/offline-tiles/cache/:code` removes just one country's cached
 extract, e.g. to force a clean re-download of it without touching the
-rest of the cache.
+rest of the cache. `GET /api/offline-tiles/cache` lists what's
+actually in that cache right now (code/size/Last-Modified per file) —
+not the same thing as offline.pmtiles' own `meta.sources`, which only
+reflects the last successful build: a country can be cached without
+ever having been built (ticked, not yet Applied) or built from without
+still being selected (unticked since). The region picker shows this
+next to each country's checkbox, with its own delete button, rather
+than only offering "delete everything" on the whole offline.pmtiles
+row.
 
 Each successful build also writes a small sidecar (`offline.pmtiles.
 meta.json` — each source country's code/Geofabrik slug/Last-Modified;