feat(layout): apply brand-navy header with accent strip

- Replace bg-surface border-b with bg-brand-navy (always #012851)
- Add 4px bg-accent strip above the nav bar
- Remove border-r separator from language switcher wrapper
- Pass inverted prop to LanguageSwitcher for white text on dark bg

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

frontend/src/routes/+layout.svelte

@@ -35,7 +35,8 @@ const userInitials = $derived.by(() => {
 
 <div class="min-h-screen bg-canvas" data-hydrated={hydrated || undefined}>
 	{#if !isAuthPage}
-		<header class="sticky top-0 z-50 border-b border-line-2 bg-surface">
+		<header class="sticky top-0 z-50 bg-brand-navy">
+			<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-16 justify-between">
 					<!-- Logo & Nav -->
@@ -45,9 +46,9 @@ const userInitials = $derived.by(() => {
 					<div class="flex items-center gap-3">
 						<!-- Language selector (desktop only — mobile lives in nav drawer) -->
 						<div
-							class="hidden items-center gap-1 border-r border-line pr-3 sm:flex [&_button]:px-1.5 [&_button]:py-1 [&_button]:text-xs"
+							class="hidden items-center gap-1 pr-3 sm:flex [&_button]:px-1.5 [&_button]:py-1 [&_button]:text-xs"
 						>
-							<LanguageSwitcher />
+							<LanguageSwitcher inverted />
 						</div>
 
 						<!-- Theme toggle -->