Swap flag emoji for real (square, bordered) SVG flags

Emoji flags are font glyphs, not shapes — no way to resize, border, or
tone them down to fit this app's controlled, hand-drawn-icon look
(same reasoning the topbar readout already settled earlier this
session). Vendored flag-icons' pre-cropped square SVGs (MIT) instead,
framed in a small rounded-square badge — the same convention
icons.js's MAP_MARKER_ICON already uses for framing a glyph, not a new
visual language.

docs/third-party-notices.adoc

@@ -55,6 +55,20 @@ overview's polar pan limit, but MapLibre 6's `Map` no longer exposes
 `.transform` at all — guarded to skip that (cosmetic) adjustment
 instead of throwing when it's absent.
 
+Also vendored under `public/vendor/` — MIT-licensed (© 2013 Panayiotis
+Lipiridis), attribution required (this notice satisfies that):
+
+[cols="1,1,2"]
+|===
+| Library | Version | Source
+
+| flag-icons (just the square `1x1/` crop, only the 39 European
+countries `lib/maptool/data/europe_countries.geojson` lists — see
+`public/js/offline_data.js`)
+| 7.5.0
+| https://github.com/lipis/flag-icons
+|===
+
 Offline map text labels need font glyphs too (see
 `docs/architecture.adoc`), vendored under `public/vendor/fonts/` —
 SIL Open Font License, not BSD-3-Clause like the above: