chore: 安装 Playwright + 配置 E2E 基础框架

- 安装 @playwright/test 1.60.0
- 创建 e2e/playwright.config.ts（webServer 自动启动 vite dev）
- 创建 e2e/mocks/ipc.ts（Tauri IPC mock）
- 新增 npm run test:e2e 脚本

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

e2e/playwright.config.ts

@@ -0,0 +1,14 @@
+import { defineConfig } from '@playwright/test';
+
+export default defineConfig({
+  testDir: './tests',
+  timeout: 10000,
+  use: {
+    baseURL: 'http://localhost:5173',
+  },
+  webServer: {
+    command: 'npm run dev',
+    url: 'http://localhost:5173',
+    reuseExistingServer: true,
+  },
+});