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/css/haus-schleusingen.css

@@ -1568,20 +1568,22 @@ footer {
 
 .locale-switcher .flag {
   width: 24px;
-  height: 16px;
+  height: 18px;
   flex: 0 0 24px;
   border-radius: 2px;
   box-shadow: 0 0 0 1px rgb(0 0 0 / 15%);
   display: block;
+  object-fit: cover;
 }
 
 .locale-switcher__option .flag {
   width: 24px;
-  height: 16px;
+  height: 18px;
   flex: 0 0 24px;
   border-radius: 2px;
   box-shadow: 0 0 0 1px rgb(0 0 0 / 15%);
   display: block;
+  object-fit: cover;
 }
 
 .locale-switcher__label {

public/img/flags/de.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-de" viewBox="0 0 640 480">
+  <path fill="#fc0" d="M0 320h640v160H0z"/>
+  <path fill="#000001" d="M0 0h640v160H0z"/>
+  <path fill="red" d="M0 160h640v160H0z"/>
+</svg>

public/img/flags/en.svg

@@ -0,0 +1,7 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-gb" viewBox="0 0 640 480">
+  <path fill="#012169" d="M0 0h640v480H0z"/>
+  <path fill="#FFF" d="m75 0 244 181L562 0h78v62L400 241l240 178v61h-80L320 301 81 480H0v-60l239-178L0 64V0z"/>
+  <path fill="#C8102E" d="m424 281 216 159v40L369 281zm-184 20 6 35L54 480H0zM640 0v3L391 191l2-44L590 0zM0 0l239 176h-60L0 42z"/>
+  <path fill="#FFF" d="M241 0v480h160V0zM0 160v160h640V160z"/>
+  <path fill="#C8102E" d="M0 193v96h640v-96zM273 0v480h96V0z"/>
+</svg>

public/img/flags/ru.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-ru" viewBox="0 0 640 480">
+  <path fill="#fff" d="M0 0h640v160H0z"/>
+  <path fill="#0039a6" d="M0 160h640v160H0z"/>
+  <path fill="#d52b1e" d="M0 320h640v160H0z"/>
+</svg>

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>