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.

views/index.erb

@@ -88,3 +88,15 @@
   </div>
   <ul class="livetrack-panel__list"></ul>
 </div>
+
+<div id="offline-data-panel" class="basemap-panel" hidden>
+  <div class="basemap-panel__header">
+    <strong>Offline data</strong>
+    <button type="button" class="basemap-panel__close" aria-label="Close">&times;</button>
+  </div>
+  <!-- Two rows (map data / terrain data), each its own status text +
+       "Regenerate for current view" button — built by
+       OfflineDataControl#mount (public/js/offline_data.js), same
+       "control owns its content" story as the Overlays panel. -->
+  <div class="offline-data-panel__body"></div>
+</div>

views/layout.erb

@@ -41,6 +41,7 @@
         "/js/locate.js": "/js/locate.js?v=<%= ASSET_VERSION %>",
         "/js/measure.js": "/js/measure.js?v=<%= ASSET_VERSION %>",
         "/js/notes.js": "/js/notes.js?v=<%= ASSET_VERSION %>",
+        "/js/offline_data.js": "/js/offline_data.js?v=<%= ASSET_VERSION %>",
         "/js/overlays.js": "/js/overlays.js?v=<%= ASSET_VERSION %>",
         "/js/panels.js": "/js/panels.js?v=<%= ASSET_VERSION %>",
         "/js/plugins.js": "/js/plugins.js?v=<%= ASSET_VERSION %>",