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.

public/vendor/flag-icons/1x1/de.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-de" viewBox="0 0 512 512">
+  <path fill="#fc0" d="M0 341.3h512V512H0z"/>
+  <path fill="#000001" d="M0 0h512v170.7H0z"/>
+  <path fill="red" d="M0 170.7h512v170.6H0z"/>
+</svg>