Replace offline basemap pipeline with Planetiler, unify online/offline styling

Offline tiles were built from Protomaps' own basemap schema, needing a
completely separate hand-tuned theme (protomaps-themes-base + OFFLINE_THEME)
from the online Liberty style's ONLINE_STYLE_PATCH — the two could never be
made to match exactly no matter how much either side got tuned (today's
session found and fixed several mismatches: building opacity, park
boundaries, road casings, road widths).

Offline.pmtiles is now built with Planetiler's default OpenMapTiles profile
against Geofabrik per-country OSM extracts (osmium-merged when more than one
country is selected), the same vector schema Liberty already expects.
offlineStyle() now clones the same Liberty JSON (vendored as a local
snapshot for reliability with no internet) and applies the identical
ONLINE_STYLE_PATCH, just repointed at local PMTiles/glyphs/sprite — one
palette instead of two, correct on both sides by construction.

Also vendors Liberty's icon sprite for the first time (offline had none
before), adds Java/osmium-tool/planetiler.jar to the Docker image, and
removes the now-unused protomaps-themes-base dependency and the
theme-comparison debug page that found today's bugs.

test/test_app.rb

@@ -402,13 +402,13 @@ class AppTest < Minitest::Test
     assert_equal 409, response.status
   end
 
-  # fetch_build_url blocks on a Queue, called directly (not through the
+  # download_extract 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
     Maptool::PmtilesBuilder.set_selected_regions(Maptool::App::DATA_DIR, ["SK"])
     gate = Queue.new
-    thread = Maptool::PmtilesBuilder.build_offline(data_dir: Maptool::App::DATA_DIR, fetch_build_url: -> { gate.pop })
+    thread = Maptool::PmtilesBuilder.build_offline(data_dir: Maptool::App::DATA_DIR, download_extract: ->(_url, _dest) { gate.pop })
 
     response = request.post("/api/offline-tiles/build", **logged_in_env)
     assert_equal 409, response.status