test(design-system): assert WCAG 2.2 AA contrast for key color pairs

White on --green-dark (not --green) is the correct button background;
--green (#3D8C4A) gives only 4.16:1 which fails AA for normal-size text.

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

frontend/src/lib/design-system/contrast.test.ts

@@ -0,0 +1,51 @@
+import { describe, it, expect } from 'vitest';
+
+// WCAG 2.2 relative luminance for a single 8-bit channel value
+function channelLuminance(val: number): number {
+	const sRGB = val / 255;
+	return sRGB <= 0.04045 ? sRGB / 12.92 : ((sRGB + 0.055) / 1.055) ** 2.4;
+}
+
+function relativeLuminance(hex: string): number {
+	const r = parseInt(hex.slice(1, 3), 16);
+	const g = parseInt(hex.slice(3, 5), 16);
+	const b = parseInt(hex.slice(5, 7), 16);
+	return 0.2126 * channelLuminance(r) + 0.7152 * channelLuminance(g) + 0.0722 * channelLuminance(b);
+}
+
+function contrastRatio(hex1: string, hex2: string): number {
+	const l1 = relativeLuminance(hex1);
+	const l2 = relativeLuminance(hex2);
+	const lighter = Math.max(l1, l2);
+	const darker = Math.min(l1, l2);
+	return (lighter + 0.05) / (darker + 0.05);
+}
+
+// Design token values from app.css @theme
+const tokens = {
+	colorText: '#1C1C18',
+	colorTextMuted: '#6B6A63',
+	colorPage: '#FAFAF7',
+	colorSurface: '#F5F4EE',
+	greenDark: '#2E6E39', // button background — --green (#3D8C4A) only gives 4.16:1, fails AA
+	white: '#FFFFFF'
+};
+
+describe('WCAG 2.2 AA contrast ratios', () => {
+	it('--color-text on --color-page meets 4.5:1 (normal text)', () => {
+		expect(contrastRatio(tokens.colorText, tokens.colorPage)).toBeGreaterThanOrEqual(4.5);
+	});
+
+	it('--color-text on --color-surface meets 4.5:1 (card text)', () => {
+		expect(contrastRatio(tokens.colorText, tokens.colorSurface)).toBeGreaterThanOrEqual(4.5);
+	});
+
+	it('--color-text-muted on --color-page meets 4.5:1 (muted labels)', () => {
+		expect(contrastRatio(tokens.colorTextMuted, tokens.colorPage)).toBeGreaterThanOrEqual(4.5);
+	});
+
+	it('white on --green-dark meets 4.5:1 (primary button background)', () => {
+		// --green (#3D8C4A) only gives 4.16:1 — buttons use --green-dark (#2E6E39) instead
+		expect(contrastRatio(tokens.white, tokens.greenDark)).toBeGreaterThanOrEqual(4.5);
+	});
+});