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.

Dockerfile

@@ -3,9 +3,14 @@ FROM ruby:4.0.6-slim-bookworm
 # build-essential: puma has a native (C) extension; harmless if the
 # platform-specific gem in Gemfile.lock already has a precompiled binary
 # matching this base image, but keeps the build working if it doesn't.
-# ca-certificates/curl: needed for the go-pmtiles download below (also
-# handy for anyone debugging inside a running container).
-RUN apt-get update && apt-get install --no-install-recommends -y build-essential ca-certificates curl \
+# ca-certificates/curl: needed for the go-pmtiles/planetiler.jar downloads
+# below (also handy for anyone debugging inside a running container).
+# default-jre-headless: Planetiler is a JVM tool (see PLANETILER_VERSION
+# below) — headless since this never touches a display.
+# osmium-tool: merges multiple countries' OSM extracts into one file before
+# handing them to Planetiler (lib/maptool/pmtiles_builder.rb) — a normal
+# Debian package, no separate download/pinning needed like the other two.
+RUN apt-get update && apt-get install --no-install-recommends -y build-essential ca-certificates curl default-jre-headless osmium-tool \
   && rm -rf /var/lib/apt/lists/*
 
 # The `pmtiles` CLI (lib/maptool/pmtiles_builder.rb's "Regenerate for
@@ -26,6 +31,15 @@ RUN set -eux; \
   curl -fsSL "https://github.com/protomaps/go-pmtiles/releases/download/v${PMTILES_VERSION}/go-pmtiles_${PMTILES_VERSION}_Linux_${pmtiles_arch}.tar.gz" \
     | tar xz -C /usr/local/bin pmtiles
 
+# Planetiler (lib/maptool/pmtiles_builder.rb's offline.pmtiles build,
+# replacing an earlier Protomaps-CDN-based pipeline — see
+# docs/development.adoc) — a single JVM fat jar, no per-arch variant needed
+# unlike go-pmtiles above. Default PLANETILER_JAR path (pmtiles_builder.rb)
+# expects it here.
+ARG PLANETILER_VERSION=0.10.2
+RUN curl -fsSL "https://github.com/onthegomap/planetiler/releases/download/v${PLANETILER_VERSION}/planetiler.jar" \
+    -o /usr/local/lib/planetiler.jar
+
 WORKDIR /app
 
 COPY Gemfile Gemfile.lock ./