fix(a11y): raise interlude label contrast to WCAG AA

#6b7280 (gray-500) on #f5f4f0 yielded ~4.0:1 — below the 4.5:1 AA minimum
for normal text. Changed to #4b5563 (gray-600) which provides ~6.9:1.

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

frontend/src/routes/layout.css

@@ -147,7 +147,7 @@
 	/* Interlude (Zwischentext) — neutral warm surface with left accent border */
 	--c-interlude-bg: #f5f4f0;
 	--c-interlude-border: #a1dcd8;
-	--c-interlude-label: #6b7280;
+	--c-interlude-label: #4b5563;
 
 	/* Tag color tokens — decorative dot colors on tag chips */
 	--c-tag-sage: #5a8a6a;