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.

vitest.config.ts

@@ -0,0 +1,30 @@
+/// <reference types="vitest" />
+import { defineConfig } from "vite";
+import react from "@vitejs/plugin-react";
+import path from "path";
+
+export default defineConfig({
+  plugins: [react()],
+  test: {
+    globals: true,
+    environment: "jsdom",
+    setupFiles: ["./tests/setup.ts"],
+    include: ["tests/**/*.{test,spec}.{ts,tsx}"],
+    exclude: ["tests/e2e/**", "**/node_modules/**"],
+    coverage: {
+      provider: "v8",
+      reporter: ["text", "json", "html"],
+      include: ["src/**/*.{ts,tsx}"],
+      exclude: ["src/main.tsx", "src/**/*.d.ts"],
+    },
+    // Mock Tauri APIs
+    alias: {
+      "@tauri-apps/api": path.resolve(__dirname, "./tests/mocks/tauri-api.ts"),
+    },
+  },
+  resolve: {
+    alias: {
+      "@": path.resolve(__dirname, "./src"),
+    },
+  },
+});