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/js/offline_data.js

@@ -17,12 +17,14 @@ function formatElapsed(ms) {
   return minutes > 0 ? `${minutes}m ${seconds}s` : `${seconds}s`;
 }
 
-// "Building…" alone gave no way to tell a slow build from a stuck one —
-// elapsed time answers "is it alive" regardless of how long a given
-// selection takes.
+// "Building…" alone gave no way to tell a slow build from a stuck one, or
+// which of the current selection's sources it's actually working on right
+// now — elapsed time answers "is it alive", status.step (see
+// PmtilesBuilder.run_job) answers "doing what, and what's left".
 function formatRunning(status) {
   const elapsed = status.started_at ? ` (${formatElapsed(Date.now() - new Date(status.started_at).getTime())})` : "";
-  return `Building…${elapsed}`;
+  const step = status.step ? `: ${status.step}` : "";
+  return `Building…${elapsed}${step}`;
 }
 
 // Vendored square-cropped SVGs (public/vendor/flag-icons/, MIT, see
@@ -131,9 +133,10 @@ class RegionPicker {
 // one class covers both rather than writing the same fetch/poll logic
 // twice.
 class DataFileRow {
-  constructor({ label, statusUrl, buildUrl, buildStatusUrl, checkUpdateUrl, formatMeta }) {
+  constructor({ label, statusUrl, deleteUrl, buildUrl, buildStatusUrl, checkUpdateUrl, formatMeta }) {
     this._label = label;
     this._statusUrl = statusUrl;
+    this._deleteUrl = deleteUrl;
     this._buildUrl = buildUrl;
     this._buildStatusUrl = buildStatusUrl;
     this._checkUpdateUrl = checkUpdateUrl;
@@ -144,10 +147,15 @@ class DataFileRow {
     const row = document.createElement("div");
     row.className = "offline-data-panel__row";
     row.innerHTML = `
-      <strong>${this._label}</strong>
+      <div class="offline-data-panel__row-head">
+        <strong>${this._label}</strong>
+        <button type="button" class="offline-data-panel__delete" hidden>Delete</button>
+      </div>
       <p class="offline-data-panel__status">Checking…</p>
     `;
     this._statusEl = row.querySelector(".offline-data-panel__status");
+    this._deleteButton = row.querySelector(".offline-data-panel__delete");
+    this._deleteButton.addEventListener("click", () => this._delete());
     container.appendChild(row);
     this._refreshStatus();
   }
@@ -163,14 +171,39 @@ class DataFileRow {
 
     if (!data.available) {
       this._statusEl.textContent = "Not generated yet.";
+      this._deleteButton.hidden = true;
       return;
     }
 
     const updated = new Date(data.mtime).toLocaleString([], { dateStyle: "short", timeStyle: "short" });
     this._statusEl.textContent = `${formatBytes(data.size)} · updated ${updated}${this._formatMeta(data.meta)}`;
+    this._deleteButton.hidden = !isLoggedIn();
     this._checkUpdate();
   }
 
+  // Deletes the built file (and, for offline.pmtiles, its whole per-country
+  // download cache — see PmtilesBuilder.delete_offline_data) so the row
+  // goes back to "Not generated yet." — a manual way to reclaim space or
+  // force a completely fresh rebuild, since Apply alone never removes
+  // anything already on disk.
+  async _delete() {
+    if (!window.confirm(`Delete ${this._label.toLowerCase()}? This can't be undone.`)) return;
+
+    this._deleteButton.disabled = true;
+    try {
+      const response = await fetch(this._deleteUrl, { method: "DELETE" });
+      const data = await response.json().catch(() => ({}));
+      if (!response.ok) throw new Error(data.error || `could not delete: ${response.status}`);
+    } catch (err) {
+      window.alert(err.message);
+      return;
+    } finally {
+      this._deleteButton.disabled = false;
+    }
+
+    this._refreshStatus();
+  }
+
   // Separate from _refreshStatus's own fetch (and only called once, right
   // after — not polled): this one reaches an external service (Geofabrik
   // or Mapterhorn's manifest, see lib/maptool/pmtiles_builder.rb), so it's
@@ -249,6 +282,7 @@ export class OfflineDataControl {
       new DataFileRow({
         label: "Offline map data",
         statusUrl: "/api/offline-tiles",
+        deleteUrl: "/api/offline-tiles",
         buildUrl: "/api/offline-tiles/build",
         buildStatusUrl: "/api/offline-tiles/build-status",
         checkUpdateUrl: "/api/offline-tiles/check-update",
@@ -257,6 +291,7 @@ export class OfflineDataControl {
       new DataFileRow({
         label: "Offline terrain data",
         statusUrl: "/api/terrain-tiles",
+        deleteUrl: "/api/terrain-tiles",
         buildUrl: "/api/terrain-tiles/build",
         buildStatusUrl: "/api/terrain-tiles/build-status",
         checkUpdateUrl: "/api/terrain-tiles/check-update",