Add a country-based region picker for offline data

Replaces "regenerate for the current viewport" with what Locus/OsmAnd-
style apps give you: tick which countries you want offline (a Natural
Earth-derived checklist, public domain, vendored), Apply rebuilds
offline.pmtiles/terrain.pmtiles to cover the whole selection at once.
Multiple regions can be offline simultaneously (e.g. Slovakia + Austria
for a trip that crosses the border) — the selection itself is the
source of truth (data/offline_regions.json), not a set of per-country
files, so "remove a country" and "refresh a country" are both just
"change the selection, then Apply."

Confirmed against go-pmtiles' own source (not just its docs) before
committing to this shape: `extract --region=<geojson>` treats a
FeatureCollection's features as their union already (no dissolve/union
math needed on our side — pmtiles/region.go's UnmarshalRegion just
concatenates every feature's rings), and `merge` hard-errors on any
overlapping tile id between inputs (merge.go's prepareInputs). That
second fact rules out "extract each country separately, merge them
all" — adjacent countries' region-extracts would share border tiles,
since extraction copies whole tiles rather than clipping vector
content to the polygon. The actual fix: extract per *source archive*
(the Protomaps build, each intersecting Mapterhorn chunk), each call
covering the whole current selection via one --region= file — the
terrain pipeline already extracted-then-merged per source this way, it
just needed a multi-country region instead of a single bbox.

docs/development.adoc

@@ -85,25 +85,47 @@ size rather than assuming one, so this isn't a problem either way.
 
 A logged-in user can also generate both files above from the app
 itself, no local tooling needed — the sidebar's "Offline data" icon
-(Offline hub group) shows each file's status (size, last updated) and
-a "Regenerate for current view" button. The region is always the
-current map viewport, same "no area-picker, the visible area is the
-area" convention the Overpass POI plugin already uses below. This
-wraps exactly the manual `go-pmtiles` workflows documented above
+(Offline hub group) shows a checklist of European countries
+(`lib/maptool/data/europe_countries.geojson`, Natural Earth 110m
+admin-0, public domain) plus each file's status (size, last updated).
+Ticking countries and clicking "Apply changes" saves the selection
+(`data/offline_regions.json`, an array of ISO A2 codes) and rebuilds
+both files to cover it.
+
+There's no standalone per-country file to individually delete —
+`offline.pmtiles`/`terrain.pmtiles` are always one combined extract for
+the *whole current selection*. "Remove a country" and "refresh a
+country" are both just "change the selection, then Apply" (unticking a
+country and applying rebuilds without it; nothing to separately
+delete). Every Apply re-extracts the entire selection from scratch, not
+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). Fetches Mapterhorn's own small JSON manifest to work
-out which regional chunk(s), if any, intersect the bbox, `extract`s
-each (plus `planet.pmtiles`, always), then `merge`s them if there was
-more than one — typically seconds, not the tens of minutes an earlier
-version of this (fetching every individual Terrarium tile itself) used
-to take for a real-sized region.
+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
+`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
+each chunk is Mapterhorn's own pre-partitioned, non-overlapping source
+file (extracting the *same* source once per country and merging those,
+by contrast, would fail — adjacent countries' region-extracts share
+border tiles, since extraction copies whole tiles rather than clipping
+vector content to the polygon, and `pmtiles merge` refuses any
+overlapping tile id between inputs).
 
-A build runs in the background (`POST /api/offline-tiles/build` /
-`/api/terrain-tiles/build`, polled via the matching `*/build-status`
-route) — only one of each kind at a time; starting a second while one's
-running gets a 409.
+A build runs in the background (`GET /api/offline-regions` to list/see
+the current selection, `POST` to change it, then `POST
+/api/offline-tiles/build` / `/api/terrain-tiles/build`, polled via the
+matching `*/build-status` route) — only one of each kind at a time;
+starting a second while one's running gets a 409, as does starting
+either with nothing selected yet.
 
 == Camper-relevant POIs (Overpass)