Add in-app regeneration of offline.pmtiles and terrain.pmtiles

A logged-in user can now trigger, from a new "Offline data" sidebar
panel, what was previously a fully manual go-pmtiles workflow
(docs/development.adoc): regenerate either file for the current map
viewport, no area-picker (same "visible area is the area" convention
the Overpass POI plugin already uses).

- lib/maptool/pmtiles_builder.rb: two independent background jobs
  (Thread + Mutex-guarded status Hash, same shape as app.rb's own GNSS
  poller), each writing to a .tmp path and atomically renaming into
  place so a build never serves a partially-written file mid-run.
  offline.pmtiles is a single `pmtiles extract` against Protomaps'
  public daily basemap build (byte-range extraction, not a full
  ~120GB download). terrain.pmtiles has no equivalent public
  Terrarium-encoded archive to extract from, so it fetches individual
  Terrarium tiles itself, packs them into an intermediate MBTiles
  (new sqlite3 gem), then the same `pmtiles convert`.
- 4 new app.rb routes (POST .../build, GET .../build-status) plus
  size/mtime added to the existing availability endpoints.
- public/js/offline_data.js: new sidebar panel, polling build-status
  the same self-rescheduling-setTimeout shape livetrack.js already
  uses for its own refresh loop.
- Both network-heavy pieces (the Protomaps build index, per-tile
  fetches) are injectable, so test/test_pmtiles_builder.rb exercises
  the actual build pipeline against a fake `pmtiles` binary without
  hitting a real network, matching this suite's existing
  keep-it-offline convention for external services.

public/css/style.css

@@ -397,6 +397,49 @@ html, body {
   visibility: visible;
 }
 
+.offline-data-panel__body {
+  padding: 8px 12px;
+}
+
+.offline-data-panel__row + .offline-data-panel__row {
+  margin-top: 10px;
+  padding-top: 10px;
+  border-top: 1px solid #eee;
+}
+
+.offline-data-panel__row-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 8px;
+}
+
+.offline-data-panel__build {
+  flex-shrink: 0;
+  border: 1px solid #ccc;
+  border-radius: 4px;
+  background: #fff;
+  padding: 4px 8px;
+  font: inherit;
+  font-size: 12px;
+  cursor: pointer;
+}
+
+.offline-data-panel__build:hover {
+  background: #f0f0f0;
+}
+
+.offline-data-panel__build:disabled {
+  opacity: 0.5;
+  cursor: default;
+}
+
+.offline-data-panel__status {
+  margin: 6px 0 0;
+  color: #666;
+  font-size: 12px;
+}
+
 .gpx-panel {
   position: fixed;
   top: 0;