feat: add theme utility with applyTheme and isDarkMode

Adds src/lib/utils/theme.svelte.ts with applyTheme() (sets data-theme
attribute and meta theme-color) and getIsDarkMode(). Uses a plain
variable instead of $state since Vitest's node environment does not
compile Svelte 5 runes in .svelte.ts files. Adds jsdom dev dependency
and $app/environment alias in vitest config to support DOM-level unit
tests; fixes preferences.test.ts to include the theme default added
in Task 2.

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

src/lib/__mocks__/app-environment.ts

@@ -0,0 +1,5 @@
+// Mock for $app/environment used in Vitest unit tests
+export const browser = true;
+export const dev = false;
+export const building = false;
+export const version = 'test';

src/lib/stores/preferences.test.ts

@@ -4,6 +4,7 @@ import { DEFAULT_PREFERENCES, BOOLEAN_PREF_KEYS } from '$lib/types/preferences.j
 describe('preferences defaults', () => {
 	it('should have correct default values', () => {
 		expect(DEFAULT_PREFERENCES).toEqual({
+			theme: 'system',
 			defaultNoteMode: 'richtext',
 			defaultNoteColor: 'default',
 			hideFooter: false,

src/lib/utils/theme.svelte.ts

@@ -0,0 +1,42 @@
+import { browser } from '$app/environment';
+
+const LIGHT_THEME_COLOR = '#f0e6d3';
+const DARK_THEME_COLOR = '#1a1715';
+
+let darkMode = false;
+
+export function getIsDarkMode(): boolean {
+	return darkMode;
+}
+
+export function applyTheme(theme: 'system' | 'light' | 'dark'): void {
+	if (!browser) return;
+
+	// Validate input
+	if (theme !== 'system' && theme !== 'light' && theme !== 'dark') {
+		theme = 'system';
+	}
+
+	// Resolve system preference
+	let resolved = theme;
+	if (theme === 'system') {
+		const prefersDark =
+			typeof window.matchMedia === 'function' &&
+			window.matchMedia('(prefers-color-scheme: dark)').matches;
+		resolved = prefersDark ? 'dark' : 'light';
+	}
+
+	const isDark = resolved === 'dark';
+	darkMode = isDark;
+
+	if (isDark) {
+		document.documentElement.setAttribute('data-theme', 'dark');
+	} else {
+		document.documentElement.removeAttribute('data-theme');
+	}
+
+	const meta = document.querySelector('meta[name="theme-color"]');
+	if (meta) {
+		meta.setAttribute('content', isDark ? DARK_THEME_COLOR : LIGHT_THEME_COLOR);
+	}
+}

src/lib/utils/theme.test.ts

@@ -0,0 +1,70 @@
+// @vitest-environment jsdom
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+
+// Mock $app/environment
+vi.mock('$app/environment', () => ({ browser: true }));
+
+describe('applyTheme', () => {
+	let applyTheme: (theme: 'system' | 'light' | 'dark') => void;
+
+	beforeEach(async () => {
+		// Reset DOM
+		document.documentElement.removeAttribute('data-theme');
+		const meta = document.querySelector('meta[name="theme-color"]');
+		if (meta) meta.remove();
+		const metaEl = document.createElement('meta');
+		metaEl.setAttribute('name', 'theme-color');
+		metaEl.setAttribute('content', '#f0e6d3');
+		document.head.appendChild(metaEl);
+
+		// Reset module state
+		vi.resetModules();
+		const mod = await import('$lib/utils/theme.svelte.js');
+		applyTheme = mod.applyTheme;
+	});
+
+	it('sets data-theme="dark" for dark mode', () => {
+		applyTheme('dark');
+		expect(document.documentElement.getAttribute('data-theme')).toBe('dark');
+	});
+
+	it('removes data-theme for light mode', () => {
+		document.documentElement.setAttribute('data-theme', 'dark');
+		applyTheme('light');
+		expect(document.documentElement.hasAttribute('data-theme')).toBe(false);
+	});
+
+	it('updates meta theme-color for dark mode', () => {
+		applyTheme('dark');
+		const meta = document.querySelector('meta[name="theme-color"]');
+		expect(meta?.getAttribute('content')).toBe('#1a1715');
+	});
+
+	it('updates meta theme-color for light mode', () => {
+		applyTheme('dark');
+		applyTheme('light');
+		const meta = document.querySelector('meta[name="theme-color"]');
+		expect(meta?.getAttribute('content')).toBe('#f0e6d3');
+	});
+
+	it('falls back to system for unknown values', () => {
+		// matchMedia returns false by default in jsdom → resolves to light
+		applyTheme('invalid' as 'system' | 'light' | 'dark');
+		expect(document.documentElement.hasAttribute('data-theme')).toBe(false);
+	});
+
+	it('resolves system preference to dark when prefers-color-scheme is dark', () => {
+		Object.defineProperty(window, 'matchMedia', {
+			writable: true,
+			value: vi.fn().mockImplementation((query: string) => ({
+				matches: query === '(prefers-color-scheme: dark)',
+				media: query,
+				addEventListener: vi.fn(),
+				removeEventListener: vi.fn(),
+				dispatchEvent: vi.fn()
+			}))
+		});
+		applyTheme('system');
+		expect(document.documentElement.getAttribute('data-theme')).toBe('dark');
+	});
+});