Fix Planetiler tmp-file format detection, add build progress and delete

Root cause of "planetiler exited with status 1: ... Unsupported format
tmp" (and likely the earlier unexplained "pmtiles exited with status
1"): Planetiler/go-pmtiles pick their output archive format from the
file extension, and the atomic-rename tmp files were named
"offline.pmtiles.tmp"/"terrain.pmtiles.tmp" — ".tmp" as the extension,
not ".pmtiles". Renamed to "offline.tmp.pmtiles"/"terrain.tmp.pmtiles".

Also, since build-status only ever said "running since X" with no way
to tell what it was actually doing:
- GET .../build-status now includes a `step` string ("fetching PL
  (3/4) — done: SK, CZ — pending: HU", "tiling with planetiler", ...)
  while a build is in progress.
- DELETE /api/offline-tiles / /api/terrain-tiles clear a built file,
  its sidecar meta, and (for offline) the whole per-country download
  cache — a manual "reclaim space" / "start over" action the region
  picker's Apply never did on its own. DELETE
  /api/offline-tiles/cache/:code clears just one country's cached
  extract. All three are 409 while that kind's build is running, and
  surfaced as a small "Delete" button on each row in the Offline data
  panel.

public/css/style.css

@@ -472,6 +472,33 @@ html, body {
   border-top: 1px solid #eee;
 }
 
+.offline-data-panel__row-head {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 8px;
+}
+
+.offline-data-panel__delete {
+  border: 1px solid #ccc;
+  border-radius: 4px;
+  background: #fff;
+  padding: 2px 8px;
+  font: inherit;
+  font-size: 11px;
+  color: #a33;
+  cursor: pointer;
+}
+
+.offline-data-panel__delete:hover {
+  background: #fdecec;
+}
+
+.offline-data-panel__delete:disabled {
+  opacity: 0.5;
+  cursor: default;
+}
+
 .offline-data-panel__status {
   margin: 6px 0 0;
   color: #666;