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.

docs/development.adoc

@@ -23,40 +23,31 @@ explicit cache headers, so an edit to `public/` (or `views/`) can
 appear not to have taken effect on the next reload. Force a real
 reload (e.g. Ctrl/Cmd+Shift+R) if that happens.
 
-`/theme_preview.html` is a standalone debug page (not linked from the
-app itself) for comparing offline base-map themes side by side: a bare
-`protomaps-themes-base` theme, an iframe of the real app's offline
-style, and one of its online style, with pan/zoom kept in sync across
-all three via the same URL-hash mechanism `map.js` already uses for
-shareable links. It's a plain static file, so nothing gates it to
-development — it's reachable on a production instance too, same as any
-other file under `public/`. Harmless if it stays that way (it shows
-nothing `/` doesn't already), but worth knowing if that's ever not the
-case.
-
 == Offline map data
 
 To enable the offline style, place a PMTiles file at
 `data/offline.pmtiles` (see `docs/architecture.adoc` for how it is
-served and styled). Logged-in users can generate this from the app
-itself — see "In-app regeneration" below — or it can be built by hand:
+served and styled — the same OpenMapTiles schema/style the online map
+uses, just from local data). Logged-in users can generate this from
+the app itself — see "In-app regeneration" below — or it can be built
+by hand:
 
-* Extract a region from Protomaps' own daily full-planet basemap build
-  (https://build-metadata.protomaps.dev/builds.json[build index],
-  files at `https://build.protomaps.com/<key>`, ~120GB, ODbL/OSM
-  attribution) with the
-  https://github.com/protomaps/go-pmtiles[go-pmtiles] CLI's `extract`
-  subcommand, which pulls only the byte ranges covering a bbox over
-  HTTP range requests rather than the whole file:
-  `pmtiles extract https://build.protomaps.com/<key> data/offline.pmtiles --bbox=west,south,east,north --maxzoom=15`.
-  (The old `app.protomaps.com` extract-builder UI this section used to
-  point at has since redirected elsewhere — this is the same
-  underlying mechanism, just the CLI instead of a web UI.)
-* To convert some other vector tile source (e.g. an `.mbtiles` file)
-  into PMTiles instead, use the same CLI's `convert` subcommand:
-  `pmtiles convert in.mbtiles data/offline.pmtiles`. Its schema is
-  what `protomaps-themes-base` (the styling library used for the
-  offline style) expects.
+* Download one or more country extracts from
+  https://download.geofabrik.de[Geofabrik] (`.osm.pbf`, ODbL/OSM
+  attribution) — e.g.
+  `curl -O https://download.geofabrik.de/europe/slovakia-latest.osm.pbf`.
+* If more than one country, merge them with
+  https://github.com/osmcode/osmium-tool[osmium-tool]:
+  `osmium merge slovakia-latest.osm.pbf austria-latest.osm.pbf -o merged.osm.pbf`
+  (each Geofabrik file is already clipped to that country's real
+  border, so no separate bbox/polygon clip step is needed either
+  before or after — skip this step entirely for a single country).
+* Tile it with https://github.com/onthegomap/planetiler[Planetiler]'s
+  default profile, which *is* OpenMapTiles — no `--profile` flag
+  needed — writing PMTiles directly:
+  `java -jar planetiler.jar --osm-path=merged.osm.pbf --output=data/offline.pmtiles`
+  (needs Java 21+; `planetiler.jar` from
+  https://github.com/onthegomap/planetiler/releases/latest[the latest release]).
 
 Once the file is in place, `GET /api/offline-tiles` reports
 `available: true` and the map defaults to the offline style.
@@ -113,14 +104,15 @@ just what changed — fine given extraction is fast (byte-range requests
 against remote archives, not per-tile fetches), but worth revisiting if
 someone selects half of Europe at once and finds it slow.
 
-This wraps exactly the manual `go-pmtiles` workflows documented above
-(`lib/maptool/pmtiles_builder.rb`) — the server needs the
-https://github.com/protomaps/go-pmtiles[go-pmtiles] CLI installed and
-on `PATH` (override with the `PMTILES_BIN` env var if it lives
-somewhere else), using `--region=<geojson>` (a `FeatureCollection` of
-the selected countries — go-pmtiles treats multiple features as their
-union already, no union/dissolve math needed on this side) rather than
-`--bbox=` for precise, per-country extraction. terrain.pmtiles extracts
+This wraps exactly the manual workflows documented above
+(`lib/maptool/pmtiles_builder.rb`). offline.pmtiles downloads each
+selected country's Geofabrik extract, `osmium merge`s them if there's
+more than one, then tiles the result with Planetiler — the server
+needs `osmium` and `java` + `planetiler.jar` installed and on `PATH`
+(override with the `OSMIUM_BIN`/`JAVA_BIN`/`PLANETILER_JAR` env vars if
+they live somewhere else; the Docker image installs all three, see
+`Dockerfile`). terrain.pmtiles still uses `go-pmtiles`
+(`PMTILES_BIN`) against Mapterhorn, unrelated to this — it extracts
 `planet.pmtiles` (always) plus whichever Mapterhorn regional chunks
 intersect the selection, `merge`s them if there was more than one —
 always safe regardless of how many/which countries are selected, since
@@ -139,18 +131,20 @@ starting a second while one's running gets a 409, as does starting
 either with nothing selected yet.
 
 Each successful build also writes a small sidecar (`offline.pmtiles.
-meta.json` / `terrain.pmtiles.meta.json` — the Protomaps build date used,
-or the Mapterhorn source names/md5sums) recording what was actually
-built, since the `.pmtiles` file itself carries no such record. `GET
-/api/offline-tiles`/`/api/terrain-tiles` include it as `meta`, shown in
-the panel next to the existing size/updated line. `GET .../check-update`
-compares that recorded build against the live upstream index/manifest
-(without rebuilding anything) and reports `update_available: true/false/
-null` — `null` means "don't know" (no local build yet, or the upstream
-check itself failed), kept as its own route rather than folded into the
-plain status endpoints above since those are on the page-load hot path
-(the topbar's offline-status dot) and this one reaches an external
-service, only worth doing when the Offline data panel is actually open.
+meta.json` — each source country's code/Geofabrik slug/Last-Modified;
+`terrain.pmtiles.meta.json` — the Mapterhorn source names/md5sums)
+recording what was actually built, since the `.pmtiles` file itself
+carries no such record. `GET /api/offline-tiles`/`/api/terrain-tiles`
+include it as `meta`, shown in the panel next to the existing
+size/updated line. `GET .../check-update` compares that recorded build
+against each source's live upstream state (a Geofabrik `HEAD` per
+country for offline, the Mapterhorn manifest for terrain — without
+rebuilding anything) and reports `update_available: true/false/null` —
+`null` means "don't know" (no local build yet, or a check itself
+failed), kept as its own route rather than folded into the plain
+status endpoints above since those are on the page-load hot path (the
+topbar's offline-status dot) and this one reaches an external service,
+only worth doing when the Offline data panel is actually open.
 
 == Camper-relevant POIs (Overpass)