test: add E2E tests for dark mode toggle and persistence

tests/e2e/dark-mode.spec.ts

@@ -0,0 +1,72 @@
+import { test, expect } from './helpers/fixtures.js';
+
+test.describe('Dark mode', () => {
+	test('Scenario: Theme toggle persists selection across page reload', async ({
+		authenticatedPage: page
+	}) => {
+		// Given the user is on the preferences page
+		await page.goto('/settings/preferences');
+
+		// When the user selects dark theme
+		await page.getByTestId('pref-theme-dark').click();
+
+		// Then the page applies dark theme
+		await expect(page.locator('html')).toHaveAttribute('data-theme', 'dark');
+
+		// When the page is reloaded
+		await page.reload();
+
+		// Then the dark theme persists without flash
+		await expect(page.locator('html')).toHaveAttribute('data-theme', 'dark');
+		await expect(page.getByTestId('pref-theme-dark')).toHaveClass(/font-medium/);
+	});
+
+	test('Scenario: Light mode removes dark theme attribute', async ({
+		authenticatedPage: page
+	}) => {
+		// Given dark theme is active
+		await page.goto('/settings/preferences');
+		await page.getByTestId('pref-theme-dark').click();
+		await expect(page.locator('html')).toHaveAttribute('data-theme', 'dark');
+
+		// When the user selects light theme
+		await page.getByTestId('pref-theme-light').click();
+
+		// Then the dark theme attribute is removed
+		await expect(page.locator('html')).not.toHaveAttribute('data-theme');
+	});
+
+	test('Scenario: System mode follows OS dark preference', async ({
+		authenticatedPage: page
+	}) => {
+		// Given the OS prefers dark mode
+		await page.emulateMedia({ colorScheme: 'dark' });
+
+		// When the user selects system theme
+		await page.goto('/settings/preferences');
+		await page.getByTestId('pref-theme-system').click();
+
+		// Then the page applies dark theme from system preference
+		// Use toHaveAttribute with timeout to handle async $effect execution
+		await expect(page.locator('html')).toHaveAttribute('data-theme', 'dark', { timeout: 5000 });
+	});
+
+	test('Scenario: Note cards use dark colors in dark mode', async ({
+		authenticatedPage: page
+	}) => {
+		// Given dark theme is active
+		await page.goto('/settings/preferences');
+		await page.getByTestId('pref-theme-dark').click();
+
+		// When viewing the notes page
+		await page.goto('/');
+
+		// Then note cards should have dark background colors
+		const noteCard = page.getByTestId('note-card').first();
+		if (await noteCard.isVisible()) {
+			const bg = await noteCard.evaluate((el) => getComputedStyle(el).backgroundColor);
+			// Dark default color is #2a2520 = rgb(42, 37, 32)
+			expect(bg).not.toBe('rgb(250, 245, 235)'); // Not the light default
+		}
+	});
+});