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.

public/js/offline_data.js

@@ -37,13 +37,13 @@ function flagIconUrl(code) {
   return `/vendor/flag-icons/1x1/${code.toLowerCase()}.svg`;
 }
 
-// meta.source is a Protomaps build filename like "20260910.pmtiles" — the
-// leading 8 digits are its build date.
+// meta.sources is one entry per country the current build covers (see
+// lib/maptool/pmtiles_builder.rb) — listing the codes is more useful here
+// than a count, since they're exactly what RegionPicker's checklist above
+// already shows.
 function formatOfflineMeta(meta) {
   if (!meta) return "";
-  const match = meta.source.match(/^(\d{4})(\d{2})(\d{2})/);
-  const date = match ? `${match[1]}-${match[2]}-${match[3]}` : meta.source;
-  return ` · Protomaps ${date}`;
+  return ` · ${meta.sources.map((s) => s.code).join(", ")}`;
 }
 
 function formatTerrainMeta(meta) {
@@ -172,9 +172,9 @@ class DataFileRow {
   }
 
   // Separate from _refreshStatus's own fetch (and only called once, right
-  // after — not polled): this one reaches an external service (Protomaps'
-  // build index or Mapterhorn's manifest, see
-  // lib/maptool/pmtiles_builder.rb), so it's kept off the hot status path
+  // after — not polled): this one reaches an external service (Geofabrik
+  // or Mapterhorn's manifest, see lib/maptool/pmtiles_builder.rb), so it's
+  // kept off the hot status path
   // and just silently skipped if it fails, same as everywhere else this
   // app degrades gracefully when an optional network check doesn't answer.
   async _checkUpdate() {