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.

test/test_helper.rb

@@ -66,6 +66,7 @@ module AppTestHelper
   def teardown
     FileUtils.rm_rf(Dir.glob(File.join(Maptool::App::DATA_DIR, "gpx", "*")))
     File.write(File.join(Maptool::App::DATA_DIR, "livetrack.json"), "[]")
+    FileUtils.rm_f(File.join(Maptool::App::DATA_DIR, "offline_regions.json"))
     Maptool::PmtilesBuilder.reset!
     super
   end