fix(flags): replace hand-coded inline SVGs with official flag-icons assets

The previous inline flag SVGs were visually broken — most notably the
'en' Union Jack, which was reduced to a single X plus a cross and did
not resemble the real flag at all. The 'de' and 'ru' stripes also had
slight off-by-pixel rounding errors.

Switched to lipis/flag-icons (CC-BY 4.0) shipped as static files under
public/img/flags/. These are the canonical, professionally-designed
flag icons with correct proportions and all the details of the real
flags. Loaded via plain <img> tags (no JS, no external CDN at
runtime, no FOUC, no extra request after the page is cached).

Locale code mapping: en -> gb (per ADR-002, en = en-GB). Unknown
locales fall back to a 1x1 transparent gif so the layout stays
intact.

public/img/flags/uk.svg

@@ -0,0 +1,6 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-ua" viewBox="0 0 640 480">
+  <g fill-rule="evenodd" stroke-width="1pt">
+    <path fill="gold" d="M0 0h640v480H0z"/>
+    <path fill="#0057b8" d="M0 0h640v240H0z"/>
+  </g>
+</svg>