fix(dark-mode): use bg-header on layout header; set --c-header to brand-navy

- Change header --c-header dark value from #01335e to #012851 (brand navy):
  #01335e gave 4.3:1 with ink-3 (WCAG AA fail); #012851 gives 4.99:1 (pass)
- Switch header element from bg-surface to bg-header so dark mode uses the
  independent --c-header token instead of inheriting the surface background
- Fix both dark blocks (media query and manual override) to stay in sync

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

frontend/e2e/theme.spec.ts

@@ -70,8 +70,8 @@ test.describe('Theme toggle', () => {
 			const header = document.querySelector('header');
 			return header ? getComputedStyle(header).backgroundColor : null;
 		});
-		// --c-header in dark mode = #01335e → rgb(1, 51, 94)
-		expect(headerBg).toBe('rgb(1, 51, 94)');
+		// --c-header in dark mode = #012851 (brand navy) → rgb(1, 40, 81)
+		expect(headerBg).toBe('rgb(1, 40, 81)');
 	});
 
 	test('color-scheme is dark when data-theme=dark is set', async ({ page }) => {

frontend/src/routes/+layout.svelte

@@ -35,7 +35,7 @@ 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 border-b border-line-2 bg-header">
 			<div class="mx-auto max-w-7xl px-4 sm:px-6 lg:px-8">
 				<div class="flex h-16 justify-between">
 					<!-- Logo & Nav -->

frontend/src/routes/layout.css

@@ -127,8 +127,8 @@
 
 		--c-nav-active: rgba(180, 185, 255, 0.12);
 
-		/* Header elevated above canvas for visual prominence */
-		--c-header: #01335e;
+		/* Header at brand-navy: 4.99:1 with ink-3 (WCAG AA ✓), visually above canvas */
+		--c-header: #012851;
 
 		--c-pdf-bg: #010e1e;
 		--c-pdf-ctrl: #011526;
@@ -161,8 +161,8 @@
 
 	--c-nav-active: rgba(180, 185, 255, 0.12);
 
-	/* Header elevated above canvas for visual prominence */
-	--c-header: #01335e;
+	/* Header at brand-navy: 4.99:1 with ink-3 (WCAG AA ✓), visually above canvas */
+	--c-header: #012851;
 
 	--c-pdf-bg: #010e1e;
 	--c-pdf-ctrl: #011526;