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/mocks/tauri-api.ts

@@ -0,0 +1,78 @@
+/**
+ * Mock implementation of @tauri-apps/api for testing
+ *
+ * This allows tests to run without a Tauri runtime.
+ */
+
+import { vi } from "vitest";
+
+// Store for mock responses - tests can override these
+export const mockResponses: Record<string, unknown> = {};
+
+// Mock invoke function
+export const invoke = vi.fn(async (cmd: string, _args?: unknown) => {
+  if (cmd in mockResponses) {
+    return mockResponses[cmd];
+  }
+
+  // Default responses
+  switch (cmd) {
+    case "greet":
+      return "Hello from mock Tauri!";
+    case "get_lore_status":
+      return {
+        last_sync: null,
+        is_healthy: true,
+        message: "Mock lore status",
+      };
+    default:
+      throw new Error(`Mock not implemented for command: ${cmd}`);
+  }
+});
+
+// Helper to set mock responses in tests
+export function setMockResponse(cmd: string, response: unknown): void {
+  mockResponses[cmd] = response;
+}
+
+// Helper to reset all mocks
+export function resetMocks(): void {
+  invoke.mockClear();
+  Object.keys(mockResponses).forEach((key) => delete mockResponses[key]);
+}
+
+// Mock event listener
+export const listen = vi.fn(
+  async (_event: string, _handler: (payload: unknown) => void) => {
+    // Return unlisten function
+    return vi.fn();
+  }
+);
+
+// Mock event emitter
+export const emit = vi.fn(async (_event: string, _payload?: unknown) => {});
+
+// Core module exports
+export const core = {
+  invoke,
+};
+
+// Event module exports
+export const event = {
+  listen,
+  emit,
+};
+
+// Window module mock
+export const window = {
+  getCurrent: vi.fn(() => ({
+    label: "main",
+    listen: vi.fn(),
+    emit: vi.fn(),
+    close: vi.fn(),
+    hide: vi.fn(),
+    show: vi.fn(),
+    isVisible: vi.fn(async () => true),
+    setTitle: vi.fn(),
+  })),
+};