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>

package.json

@@ -20,6 +20,7 @@
     "dep:check": "depcruise src/app --config .dependency-cruiser.js",
     "dep:graph": "bash scripts/dep-graph.sh",
     "gen:snippets": "node scripts/gen-snippets.mjs",
+    "serve:i18n": "ng build --localize && node scripts/serve-i18n.mjs",
     "ci": "bash scripts/ci-local.sh",
     "e2e": "playwright test",
     "extract-i18n": "ng extract-i18n --output-path src/locale"