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/is.svg

@@ -0,0 +1,12 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-is" viewBox="0 0 512 512">
+  <defs>
+    <clipPath id="is-a">
+      <path fill-opacity=".7" d="M85.4 0h486v486h-486z"/>
+    </clipPath>
+  </defs>
+  <g fill-rule="evenodd" stroke-width="0" clip-path="url(#is-a)" transform="translate(-90)scale(1.0535)">
+    <path fill="#003897" d="M0 0h675v486H0z"/>
+    <path fill="#fff" d="M0 189h189V0h108v189h378v108H297v189H189V297H0z"/>
+    <path fill="#d72828" d="M0 216h216V0h54v216h405v54H270v216h-54V270H0z"/>
+  </g>
+</svg>