test: add test infrastructure with Vitest and Playwright

Set up comprehensive testing infrastructure for both unit and E2E tests:

Unit Testing (Vitest):
- vitest.config.ts: jsdom environment, globals enabled
  - Path alias @tauri-apps/api -> tests/mocks/tauri-api.ts
  - Excludes tests/e2e/** to prevent Playwright collision
  - V8 coverage configured for src/**/*.{ts,tsx}
- tests/setup.ts: @testing-library/jest-dom matchers

Tauri API Mocking:
- tests/mocks/tauri-api.ts: Mock implementation of @tauri-apps/api
  - invoke(): Returns configurable mock responses
  - listen()/emit(): Event system stubs
  - setMockResponse()/resetMocks(): Test helpers
  - Enables testing React components without Tauri runtime

Component Tests:
- tests/components/App.test.tsx: Verifies App shell renders
  - "Mission Control" heading
  - "What should you be doing right now?" tagline
  - "THE ONE THING will appear here" placeholder

E2E Testing (Playwright):
- playwright.config.ts: Chromium + WebKit (Tauri uses WebKit on macOS)
  - Runs Vite dev server before tests
  - HTML reporter, trace on retry
- tests/e2e/app.spec.ts: Smoke tests for deployed app
  - Heading visible, tagline visible, dark mode applied

This dual-layer testing strategy (Vitest for speed, Playwright for
integration) follows the testing trophy: many unit, fewer E2E.

tests/setup.ts

@@ -0,0 +1,25 @@
+import "@testing-library/jest-dom/vitest";
+
+// Mock window.matchMedia for components that use media queries
+Object.defineProperty(window, "matchMedia", {
+  writable: true,
+  value: vi.fn().mockImplementation((query: string) => ({
+    matches: false,
+    media: query,
+    onchange: null,
+    addListener: vi.fn(),
+    removeListener: vi.fn(),
+    addEventListener: vi.fn(),
+    removeEventListener: vi.fn(),
+    dispatchEvent: vi.fn(),
+  })),
+});
+
+// Mock ResizeObserver for components that use it
+class MockResizeObserver {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+}
+
+window.ResizeObserver = MockResizeObserver as typeof ResizeObserver;