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/gb.svg

@@ -0,0 +1,7 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-gb" viewBox="0 0 512 512">
+  <path fill="#012169" d="M0 0h512v512H0z"/>
+  <path fill="#FFF" d="M512 0v64L322 256l190 187v69h-67L254 324 68 512H0v-68l186-187L0 74V0h62l192 188L440 0z"/>
+  <path fill="#C8102E" d="m184 324 11 34L42 512H0v-3zm124-12 54 8 150 147v45zM512 0 320 196l-4-44L466 0zM0 1l193 189-59-8L0 49z"/>
+  <path fill="#FFF" d="M176 0v512h160V0zM0 176v160h512V176z"/>
+  <path fill="#C8102E" d="M0 208v96h512v-96zM208 0v512h96V0z"/>
+</svg>