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.

test/test_app.rb

@@ -70,6 +70,8 @@ class AppTest < Minitest::Test
     assert_equal 401, request.delete("/api/gpx/folder/x").status
     assert_equal 401, request.post("/api/gpx/move").status
     assert_equal 401, request.post("/api/plugins/test-plugin/run").status
+    assert_equal 401, request.post("/api/offline-tiles/build").status
+    assert_equal 401, request.post("/api/terrain-tiles/build").status
   end
 
   # --- GPX/FIT upload and the tree ---
@@ -347,6 +349,47 @@ class AppTest < Minitest::Test
     assert_equal 502, response.status
   end
 
+  # --- pmtiles regeneration ---
+  #
+  # The full build (network + go-pmtiles) is covered in isolation, with
+  # fakes injected, by test_pmtiles_builder.rb — same "keep this offline"
+  # reasoning as the rest of this file. What's route-specific and worth
+  # covering here is the bbox validation and the 409-while-running guard,
+  # neither of which needs a real build to actually run.
+
+  def test_offline_tiles_build_status_defaults_to_idle
+    assert_equal({ "state" => "idle" }, json_body(request.get("/api/offline-tiles/build-status")))
+    assert_equal({ "state" => "idle" }, json_body(request.get("/api/terrain-tiles/build-status")))
+  end
+
+  def test_offline_tiles_build_requires_a_bbox
+    response = request.post("/api/offline-tiles/build", **logged_in_env)
+    assert_equal 400, response.status
+  end
+
+  def test_terrain_tiles_build_requires_a_bbox
+    response = request.post("/api/terrain-tiles/build", **logged_in_env)
+    assert_equal 400, response.status
+  end
+
+  # fetch_build_url blocks on a Queue, called directly (not through the
+  # route, which has no way to inject a fake) so the build is
+  # deterministically still "running" when the route call below happens.
+  def test_offline_tiles_build_is_409_when_one_is_already_running
+    gate = Queue.new
+    thread = Maptool::PmtilesBuilder.build_offline([1, 2, 3, 4], data_dir: Maptool::App::DATA_DIR, fetch_build_url: -> { gate.pop })
+
+    response = request.post(
+      "/api/offline-tiles/build",
+      params: { south: "1", west: "2", north: "3", east: "4" },
+      **logged_in_env,
+    )
+    assert_equal 409, response.status
+  ensure
+    gate.push("unused")
+    thread&.join
+  end
+
   # --- LiveTrack webhook ---
 
   def test_livetrack_webhook_requires_the_configured_secret