Show build progress instead of a static "Building…"

terrain.pmtiles can take minutes (tiles fetched one at a time) with no
way to tell a slow build from a stuck one. Now reports tiles_done/
tiles_total into the same status Hash the frontend already polls, and
the panel shows elapsed time (both files) plus tile counts (terrain
only) so the numbers visibly move every ~3s poll instead of sitting on
one static string.

test/test_pmtiles_builder.rb

@@ -113,7 +113,34 @@ class PmtilesBuilderTest < Minitest::Test
     thread.join
 
     assert_path_exists File.join(@data_dir, "terrain.pmtiles")
-    assert_equal "done", Maptool::PmtilesBuilder.terrain_status[:state]
+    status = Maptool::PmtilesBuilder.terrain_status
+    assert_equal "done", status[:state]
+    assert_operator status[:tiles_total], :>, 0
+    assert_equal status[:tiles_total], status[:tiles_done]
+  end
+
+  # fetch_tile blocks after the very first call so the "running" progress
+  # can be observed mid-flight, deterministically, rather than only ever
+  # seeing the final done/total once the (very fast, faked) build finishes.
+  def test_build_terrain_reports_progress_while_running
+    ENV["PMTILES_BIN"] = @pmtiles_ok
+    gate = Queue.new
+    first_call = true
+    fetch_tile = lambda do |_z, _x, _y|
+      gate.pop if first_call
+      first_call = false
+      "bytes"
+    end
+    thread = Maptool::PmtilesBuilder.build_terrain([48.140, 17.100, 48.141, 17.101], data_dir: @data_dir, fetch_tile: fetch_tile)
+
+    20.times { break if Maptool::PmtilesBuilder.terrain_status[:tiles_total]; sleep 0.01 }
+    status = Maptool::PmtilesBuilder.terrain_status
+    assert_equal "running", status[:state]
+    assert_operator status[:tiles_total], :>, 0
+    assert_equal 0, status[:tiles_done]
+  ensure
+    gate.push(nil)
+    thread&.join
   end
 
   def test_build_terrain_still_succeeds_when_some_tiles_are_missing