feat(i18n): CIBG "Taal instellen" language switcher

Add a language switcher matching the CIBG Taal-instellen pattern: a <nav> region
(sr-only heading + aria-label) with an endonym link per locale (lang/hreflang, the
active one aria-current + non-link), mounted right after the skip link in the shell.
Compile-time $localize means each locale is its own bundle under /<locale>/, so the
switch is a full navigation to the sibling bundle — active locale read from the baked
base href. Pure localeLinks() (+spec) builds path-preserving targets. Since `ng serve`
serves nl-only at /, add `npm run serve:i18n` (localized build + a tiny static server
with per-locale SPA fallback) so the switch is demoable. +story.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/locale/messages.en.xlf

@@ -2234,6 +2234,14 @@
           <context context-type="linenumber">48,49</context>
         </context-group>
       </trans-unit>
+      <trans-unit id="lang.navLabel" datatype="html">
+        <source>Taal / Language</source>
+        <target datatype="html">Taal / Language</target>
+      </trans-unit>
+      <trans-unit id="lang.heading" datatype="html">
+        <source>Kies een taal</source>
+        <target datatype="html">Choose a language</target>
+      </trans-unit>
       <trans-unit id="footer.tagline" datatype="html">
         <source>De Rijksoverheid. Voor Nederland.</source>
         <target datatype="html">The Government of the Netherlands.</target>