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/mk.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-mk" viewBox="0 0 512 512">
+  <path fill="#d20000" d="M0 0h512v512H0z"/>
+  <path fill="#ffe600" d="M0 0h86.8L256 246.9 425.2 0H512L0 512h86.8L256 265.1 425.2 512H512zm512 204.8v102.4L0 204.8v102.4zM204.8 0 256 219.4 307.2 0zm0 512L256 292.6 307.2 512z"/>
+  <circle cx="256" cy="256" r="82.3" fill="#ffe600" stroke="#d20000" stroke-width="18.3"/>
+</svg>