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.

TODO.adoc

@@ -8,7 +8,7 @@ independence from connectivity.
 
 == Offline data and cartography
 
-* [ ] Offline vector tiles (PMTiles/MBTiles) for Europe/world via Planetiler or MapTiler
+* [x] Offline vector tiles (PMTiles/MBTiles) for Europe/world via Planetiler or MapTiler — Geofabrik per-country extracts, `osmium merge`, Planetiler's default OpenMapTiles profile straight to PMTiles (`lib/maptool/pmtiles_builder.rb`); offline now shares the exact same style/palette as online (`ONLINE_STYLE_PATCH`) instead of a separate protomaps-schema theme
 * [x] Shaded relief / DEM layer — hillshade toggle in the sidebar, sourced from public Terrarium tiles online; reads `data/terrain.pmtiles` for offline if one's been generated (see docs/development.adoc), otherwise the toggle is online-only for now
 ** [ ] Terrain slope readout for parking (numeric, not just visual shading)
 * [ ] Contour lines from OSM for hikes and mountain passes