fix(header): address PR review blockers

- AuthHeader: remove duplicated locale logic, use <LanguageSwitcher inverted />
- Fix text-white/55 → text-white/70 in AppNav and LanguageSwitcher (WCAG AA)
- E2E: add axe accessibility checks, replace [data-hydrated] with role selectors,
  add 768px hamburger test and BRAND_NAVY comment

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/AuthHeader.svelte

@@ -1,9 +1,5 @@
 <script lang="ts">
-import { setLocale, getLocale } from '$lib/paraglide/runtime';
-
-const locales = ['DE', 'EN', 'ES'] as const;
-const localeMap = { DE: 'de', EN: 'en', ES: 'es' } as const;
-const activeLocale = $derived(getLocale().toUpperCase());
+import LanguageSwitcher from '$lib/components/LanguageSwitcher.svelte';
 </script>
 
 <header class="bg-brand-navy">
@@ -16,18 +12,7 @@ const activeLocale = $derived(getLocale().toUpperCase());
 				>
 			</a>
 			<div class="flex items-center gap-1">
-				{#each locales as locale (locale)}
-					<button
-						type="button"
-						onclick={() => setLocale(localeMap[locale])}
-						class="px-1.5 py-1 font-sans text-xs tracking-widest transition-colors focus:outline-none focus-visible:ring-2 focus-visible:ring-accent
-                            {activeLocale === locale
-							? 'font-bold text-white'
-							: 'font-normal text-white/55 hover:text-white/85'}"
-					>
-						{locale}
-					</button>
-				{/each}
+				<LanguageSwitcher inverted />
 			</div>
 		</div>
 	</div>