test(a11y): add dark mode axe + color-scheme tests for issue #166

Two failing test suites that encode the regressions this issue fixes:
- accessibility.spec.ts: axe wcag2aa in both prefers-color-scheme:dark
  and data-theme='dark' — fails because --c-ink-3:#6b7280 on #1a1a1a = 3.2:1
- theme.spec.ts: color-scheme computed property is 'dark' in dark mode
  — fails because neither dark CSS block sets color-scheme: dark

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

frontend/e2e/accessibility.spec.ts

@@ -37,6 +37,57 @@ test.describe('Accessibility — authenticated pages', () => {
 	}
 });
 
+test.describe('Accessibility — dark mode (system preference)', () => {
+	for (const { name, path } of AUTHENTICATED_PAGES) {
+		test(`${name} page has no wcag2a/wcag2aa violations in prefers-color-scheme: dark`, async ({
+			browser
+		}) => {
+			const context = await browser.newContext({
+				colorScheme: 'dark',
+				storageState: 'e2e/.auth/user.json'
+			});
+			const page = await context.newPage();
+			await page.goto(path);
+			await page.waitForSelector('[data-hydrated]');
+
+			const results = await buildAxe(page).analyze();
+
+			if (results.violations.length > 0) {
+				const summary = results.violations
+					.map((v) => `[${v.impact}] ${v.id}: ${v.description} (${v.nodes.length} node(s))`)
+					.join('\n');
+				console.log(`\nAccessibility violations on ${name} (dark/media):\n${summary}`);
+			}
+
+			await context.close();
+			expect(results.violations).toEqual([]);
+		});
+	}
+});
+
+test.describe('Accessibility — dark mode (manual toggle)', () => {
+	for (const { name, path } of AUTHENTICATED_PAGES) {
+		test(`${name} page has no wcag2a/wcag2aa violations with data-theme='dark'`, async ({
+			page
+		}) => {
+			await page.goto(path);
+			await page.waitForSelector('[data-hydrated]');
+			await page.evaluate(() => document.documentElement.setAttribute('data-theme', 'dark'));
+
+			const results = await buildAxe(page).analyze();
+
+			if (results.violations.length > 0) {
+				const summary = results.violations
+					.map((v) => `[${v.impact}] ${v.id}: ${v.description} (${v.nodes.length} node(s))`)
+					.join('\n');
+				console.log(`\nAccessibility violations on ${name} (dark/manual):\n${summary}`);
+			}
+
+			expect(results.violations).toEqual([]);
+		});
+	}
+});
+
 test.describe('Accessibility — login page', () => {
 	test.use({ storageState: { cookies: [], origins: [] } });
 

frontend/e2e/theme.spec.ts

@@ -60,6 +60,34 @@ test.describe('Theme toggle', () => {
 		await expect(page.locator('html')).toHaveAttribute('data-theme', 'dark');
 	});
 
+	test('color-scheme is dark when data-theme=dark is set', async ({ page }) => {
+		await page.goto('/');
+		await page.waitForSelector('[data-hydrated]');
+
+		await page.evaluate(() => document.documentElement.setAttribute('data-theme', 'dark'));
+
+		const colorScheme = await page.evaluate(
+			() => getComputedStyle(document.documentElement).colorScheme
+		);
+		expect(colorScheme).toBe('dark');
+	});
+
+	test('color-scheme is dark in prefers-color-scheme: dark media', async ({ browser }) => {
+		const context = await browser.newContext({
+			colorScheme: 'dark',
+			storageState: 'e2e/.auth/user.json'
+		});
+		const page = await context.newPage();
+		await page.goto('/');
+		await page.waitForSelector('[data-hydrated]');
+
+		const colorScheme = await page.evaluate(
+			() => getComputedStyle(document.documentElement).colorScheme
+		);
+		await context.close();
+		expect(colorScheme).toBe('dark');
+	});
+
 	test('saved theme is applied before first paint (no flash)', async ({ page }) => {
 		// Set dark theme in localStorage before navigating
 		await page.goto('/');