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/js/icons.js

@@ -416,6 +416,16 @@ Object.assign(POI_ICONS, {
   shop_convenience: STORE_ICON,
 });
 
+// Hand-drawn, not NPS-sourced — no "download offline map data" pictogram
+// in that set either. A tray with a downward arrow into it, same solid
+// 22x22 currentColor convention as CONTOUR_ICON/TERRAIN_SHADOW_ICON above
+// — used for the "Offline data" sidebar icon (offline_data.js).
+export const OFFLINE_DATA_ICON = `
+<svg viewBox="0 0 22 22" fill="currentColor">
+  <path d="M10,0h2v10.5l3-3l1.4,1.4L11,14.3L5.6,8.9L7,7.5l3,3V0z"/>
+  <path d="M2,15h18v2H2z M2,18h18v2H2z"/>
+</svg>`;
+
 // src/bookstore-22.svg (NPS symbol library, same source/license as
 // POI_ICONS above) — used as the fixed marker glyph for the "Wikipedia"
 // map layer (gpx.js), not part of POI_ICONS itself: that set is keyed by