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.

public/js/map.js

@@ -516,7 +516,7 @@ if (isLoggedIn()) new PluginsControl(map).mount(sidebar);
 sidebar.insertAdjacentHTML("beforeend", `<div class="sidebar-group-label" title="Offline hub"></div>`);
 layerToggle.mount(sidebar);
 layerToggle.setActive("online");
-new OfflineDataControl(map).mount(sidebar);
+new OfflineDataControl().mount(sidebar);
 
 sidebar.insertAdjacentHTML("beforeend", `<div class="sidebar-group-label" title="Navigation"></div>`);
 gnssControl.mount(sidebar);