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/lt.svg

@@ -0,0 +1,7 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-lt" viewBox="0 0 512 512">
+  <g fill-rule="evenodd" stroke-width="1pt" transform="scale(.51314 1.0322)">
+    <rect width="1063" height="708.7" fill="#006a44" rx="0" ry="0" transform="scale(.93865 .69686)"/>
+    <rect width="1063" height="236.2" y="475.6" fill="#c1272d" rx="0" ry="0" transform="scale(.93865 .69686)"/>
+    <path fill="#fdb913" d="M0 0h997.8v164.6H0z"/>
+  </g>
+</svg>