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/lib/components/LanguageSwitcher.svelte

@@ -18,7 +18,7 @@ const activeLocale = $derived(getLocale().toUpperCase());
 				? 'font-bold text-white'
 				: 'font-bold text-ink'
 			: inverted
-				? 'font-normal text-white/55 hover:text-white/85'
+				? 'font-normal text-white/70 hover:text-white'
 				: 'font-normal text-ink-3 hover:text-ink'}"
 	>
 		{locale}