fix(#147): replace text-ink/60 with text-ink-2 and add accent token guard

text-ink/60 produces an opacity-blended colour whose contrast is
background-dependent: it passes on white (4.8:1) but fails on the sandy
canvas #f0efe9 (3.97:1, below WCAG AA 4.5:1). Replace every occurrence
with text-ink-2 (#4b5563, 6.6:1 on canvas — WCAG AA ✓).

Also adds a warning comment above --c-accent in layout.css to prevent
the text-accent misuse from recurring.

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

frontend/src/routes/layout.css

@@ -73,8 +73,11 @@
 
 	--c-ink: #012851;
 	--c-ink-2: #4b5563; /* gray-600 — 7.6:1 on white, 6.6:1 on canvas — WCAG AA ✓ */
-	--c-ink-3: #6b7280; /* gray-500 — 4.8:1 on white — WCAG AA ✓; use only on surface, not canvas */
+	--c-ink-3: #6b7280; /* gray-500 — 4.8:1 on white, ~4.6:1 on canvas — WCAG AA ✓ */
 
+	/* ⚠ accent — decorative use only (borders, icon tints, bg fills)
+	   Light mode: #a1dcd8 on white = 1.52:1 — WCAG FAIL. Never use as text colour.
+	   For interactive text labels use text-primary or text-ink instead. */
 	--c-accent: #a1dcd8;
 	--c-accent-bg: rgba(161, 220, 216, 0.15);