feat: use native OS folder picker dialog in Electron mode

Electron 环境下改用系统原生文件夹选择对话框替代自定义 FolderPicker 组件，
浏览器环境保留原有 FolderPicker 作为 fallback（浏览器沙箱无法获取绝对路径）。

- electron/main.ts: 新增 `dialog:open-folder` IPC handler，调用
  `dialog.showOpenDialog` 并设置 openDirectory + createDirectory 属性，
  macOS 下支持在对话框内新建文件夹，对话框模态绑定 mainWindow
- electron/preload.ts: 在 electronAPI 中暴露 `dialog.openFolder()` 方法
- src/types/electron.d.ts: 新建全局类型声明，统一定义 window.electronAPI
  的完整类型，消除代码中散落的 `(window as any)` 类型断言
- src/hooks/useNativeFolderPicker.ts: 新建 hook 封装平台检测和原生对话框
  调用逻辑，返回 isElectron 标志和 openNativePicker 方法
- src/components/layout/ChatListPanel.tsx: 新增 openFolderPicker() 统一入口，
  Electron 走原生对话框、浏览器走 setFolderPickerOpen(true)；替换
  handleNewChat 中 4 处及文件夹按钮 onClick 共 5 处调用点；
  handleFolderSelect 改为 useCallback 并调整声明顺序避免引用前使用；
  清理 electronAPI 类型断言
- src/app/chat/[id]/page.tsx: 移除 `window as unknown as ...` 类型断言
- src/components/layout/ConnectionStatus.tsx: 移除 `(window as any)` 断言
- src/components/layout/InstallWizard.tsx: getInstallAPI() 简化为直接返回
  window.electronAPI?.install，移除 10 行手动类型定义

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

electron/main.ts

@@ -712,6 +712,17 @@ app.whenReady().then(async () => {
     return shell.openPath(folderPath);
   });
 
+  // Native folder picker dialog
+  ipcMain.handle('dialog:open-folder', async (_event, options?: { defaultPath?: string; title?: string }) => {
+    if (!mainWindow) return { canceled: true, filePaths: [] };
+    const result = await dialog.showOpenDialog(mainWindow, {
+      title: options?.title || 'Select a project folder',
+      defaultPath: options?.defaultPath || undefined,
+      properties: ['openDirectory', 'createDirectory'],
+    });
+    return { canceled: result.canceled, filePaths: result.filePaths };
+  });
+
   try {
     let port: number;
 

electron/preload.ts

@@ -10,6 +10,10 @@ contextBridge.exposeInMainWorld('electronAPI', {
   shell: {
     openPath: (folderPath: string) => ipcRenderer.invoke('shell:open-path', folderPath),
   },
+  dialog: {
+    openFolder: (options?: { defaultPath?: string; title?: string }) =>
+      ipcRenderer.invoke('dialog:open-folder', options),
+  },
   install: {
     checkPrerequisites: () => ipcRenderer.invoke('install:check-prerequisites'),
     start: (options?: { includeNode?: boolean }) => ipcRenderer.invoke('install:start', options),