fix(dark-mode): address PR #168 review blockers

- AuthHeader: bg-brand-navy → bg-header (semantic token, responds to dark mode)
- header.spec.ts: add forgot-password AuthHeader tests (bg + axe)
- header.spec.ts: fix BRAND_NAVY comment — references --c-header, not --c-primary

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

frontend/src/routes/AuthHeader.svelte

@@ -2,7 +2,7 @@
 import LanguageSwitcher from '$lib/components/LanguageSwitcher.svelte';
 </script>
 
-<header class="bg-brand-navy">
+<header class="bg-header">
 	<div class="h-1 bg-accent"></div>
 	<div class="mx-auto max-w-7xl px-4 sm:px-6 lg:px-8">
 		<div class="flex h-12 items-center justify-between">