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.

docker-compose.yaml

@@ -60,7 +60,11 @@ services:
       # docs/development.adoc for how the deploy job treats this
       # directory). A private plugin (app.rb's PLUGINS_DIRS) also belongs
       # under here, at data/plugins/<name>/ — nothing further to mount,
-      # this one bind already covers the whole data/ tree.
+      # this one bind already covers the whole data/ tree. An offline.pmtiles
+      # build's own scratch work (Dir.mktmpdir, lib/maptool/pmtiles_builder.rb)
+      # happens outside this mount, in the container's own filesystem — a
+      # large country's .osm.pbf plus Planetiler's working set can be
+      # several GB, worth knowing if the host is ever short on space there.
       - ./data:/app/data
 
   # Fakes a GNSS fix (a loop around Bratislava, see