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.

playwright.config.ts

@@ -0,0 +1,43 @@
+import { defineConfig, devices } from "@playwright/test";
+
+/**
+ * Playwright configuration for Mission Control E2E tests
+ *
+ * For Tauri desktop app testing, we use the webview URL directly
+ * rather than launching the full native app wrapper.
+ */
+export default defineConfig({
+  testDir: "./tests/e2e",
+  fullyParallel: true,
+  forbidOnly: !!process.env.CI,
+  retries: process.env.CI ? 2 : 0,
+  workers: process.env.CI ? 1 : undefined,
+  reporter: "html",
+
+  use: {
+    // Base URL for webview testing
+    baseURL: "http://localhost:1420",
+    trace: "on-first-retry",
+    screenshot: "only-on-failure",
+  },
+
+  projects: [
+    {
+      name: "chromium",
+      use: { ...devices["Desktop Chrome"] },
+    },
+    // Note: Tauri uses WebKit on macOS, so we test against Safari-like behavior
+    {
+      name: "webkit",
+      use: { ...devices["Desktop Safari"] },
+    },
+  ],
+
+  // Run the Vite dev server before tests
+  webServer: {
+    command: "npm run dev",
+    url: "http://localhost:1420",
+    reuseExistingServer: !process.env.CI,
+    timeout: 120000,
+  },
+});