chore: merge main into feat/issue-166 — resolve blue header conflicts

- +layout.svelte: adopt main's blue header structure (accent stripe, no
  border-b, bg-header instead of bg-brand-navy)
- layout.css light mode: drop --c-nav-active (removed by main); set
  --c-header: #012851 (confirmed correct now that header is brand-navy)
- layout.css dark mode: drop --c-nav-active; keep navy PDF tokens and
  --c-header: #012851 from our branch

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

frontend/src/routes/layout.css

@@ -48,9 +48,6 @@
 	--color-primary: var(--c-primary);
 	--color-primary-fg: var(--c-primary-fg);
 
-	/* Nav active state */
-	--color-nav-active: var(--c-nav-active);
-
 	/* PDF viewer */
 	--color-pdf-bg: var(--c-pdf-bg);
 	--color-pdf-ctrl: var(--c-pdf-ctrl);
@@ -87,10 +84,8 @@
 	--c-primary: #012851;
 	--c-primary-fg: #ffffff;
 
-	--c-nav-active: rgba(180, 185, 255, 0.15);
-
-	/* Header matches surface in light mode; overridden in dark mode for elevation */
-	--c-header: #ffffff;
+	/* Header is brand-navy in light mode; same in dark mode for contrast compliance */
+	--c-header: #012851;
 
 	--c-pdf-bg: #ebebeb;
 	--c-pdf-ctrl: #d8d8d8;
@@ -125,8 +120,6 @@
 		--c-primary: #a1dcd8;
 		--c-primary-fg: #012851;
 
-		--c-nav-active: rgba(180, 185, 255, 0.12);
-
 		/* Header at brand-navy: 4.99:1 with ink-3 (WCAG AA ✓), visually above canvas */
 		--c-header: #012851;
 
@@ -159,8 +152,6 @@
 	--c-primary: #a1dcd8;
 	--c-primary-fg: #012851;
 
-	--c-nav-active: rgba(180, 185, 255, 0.12);
-
 	/* Header at brand-navy: 4.99:1 with ink-3 (WCAG AA ✓), visually above canvas */
 	--c-header: #012851;