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.

app.rb

@@ -223,18 +223,38 @@ module Maptool
       send_file OFFLINE_PMTILES_PATH, type: "application/octet-stream"
     end
 
-    # Regenerates offline.pmtiles for the given bbox (the current map
-    # viewport, same "visible area is the area" convention as the Overpass
-    # POI plugin) by extracting it from Protomaps' own daily full-planet
-    # basemap build — see lib/maptool/pmtiles_builder.rb. Runs in the
-    # background; poll build-status below for progress.
-    post "/api/offline-tiles/build" do
+    # Lists the selectable countries and which of them are currently
+    # selected (data/offline_regions.json) — read-only, no login needed,
+    # same tier as the plain availability checks above.
+    get "/api/offline-regions" do
+      json available: PmtilesBuilder.available_regions, selected: PmtilesBuilder.selected_regions(DATA_DIR)
+    end
+
+    # Just updates the selection — does not itself start a build, so
+    # ticking several countries doesn't fire off several rebuilds. The
+    # frontend follows this with the two build routes below once the user
+    # is done choosing.
+    post "/api/offline-regions" do
       require_login!
-      bbox = %w[south west north east].map { |key| params[key].to_s }
-      halt 400, json(error: "south, west, north, east are required") if bbox.any?(&:empty?)
+      codes = Array(params[:codes])
 
       begin
-        PmtilesBuilder.build_offline(bbox.map(&:to_f), data_dir: DATA_DIR)
+        PmtilesBuilder.set_selected_regions(DATA_DIR, codes)
+      rescue PmtilesBuilder::BuildError => e
+        halt 400, json(error: e.message)
+      end
+      json ok: true
+    end
+
+    # Regenerates offline.pmtiles for the currently selected regions (see
+    # POST /api/offline-regions above) by extracting them from Protomaps'
+    # own daily full-planet basemap build — see
+    # lib/maptool/pmtiles_builder.rb. Runs in the background; poll
+    # build-status below for progress.
+    post "/api/offline-tiles/build" do
+      require_login!
+      begin
+        PmtilesBuilder.build_offline(data_dir: DATA_DIR)
       rescue PmtilesBuilder::BuildError => e
         halt 409, json(error: e.message)
       end
@@ -260,11 +280,8 @@ module Maptool
 
     post "/api/terrain-tiles/build" do
       require_login!
-      bbox = %w[south west north east].map { |key| params[key].to_s }
-      halt 400, json(error: "south, west, north, east are required") if bbox.any?(&:empty?)
-
       begin
-        PmtilesBuilder.build_terrain(bbox.map(&:to_f), data_dir: DATA_DIR)
+        PmtilesBuilder.build_terrain(data_dir: DATA_DIR)
       rescue PmtilesBuilder::BuildError => e
         halt 409, json(error: e.message)
       end