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/components/App.test.tsx

@@ -0,0 +1,27 @@
+import { describe, it, expect } from "vitest";
+import { render, screen } from "@testing-library/react";
+import App from "@/App";
+
+describe("App", () => {
+  it("renders the main heading", () => {
+    render(<App />);
+
+    expect(screen.getByText("Mission Control")).toBeInTheDocument();
+  });
+
+  it("renders the tagline", () => {
+    render(<App />);
+
+    expect(
+      screen.getByText("What should you be doing right now?")
+    ).toBeInTheDocument();
+  });
+
+  it("renders the focus placeholder", () => {
+    render(<App />);
+
+    expect(
+      screen.getByText("THE ONE THING will appear here")
+    ).toBeInTheDocument();
+  });
+});