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>

src/types/electron.d.ts

@@ -0,0 +1,44 @@
+/**
+ * Global type declarations for the Electron preload API.
+ * Exposed via contextBridge.exposeInMainWorld('electronAPI', ...) in electron/preload.ts.
+ */
+
+interface ElectronInstallAPI {
+  checkPrerequisites: () => Promise<{
+    hasNode: boolean;
+    nodeVersion?: string;
+    hasClaude: boolean;
+    claudeVersion?: string;
+  }>;
+  start: (options?: { includeNode?: boolean }) => Promise<void>;
+  cancel: () => Promise<void>;
+  getLogs: () => Promise<string[]>;
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  onProgress: (callback: (data: any) => void) => () => void;
+}
+
+interface ElectronAPI {
+  versions: {
+    electron: string;
+    node: string;
+    chrome: string;
+  };
+  shell: {
+    openPath: (path: string) => Promise<string>;
+  };
+  dialog: {
+    openFolder: (options?: {
+      defaultPath?: string;
+      title?: string;
+    }) => Promise<{ canceled: boolean; filePaths: string[] }>;
+  };
+  install: ElectronInstallAPI;
+}
+
+declare global {
+  interface Window {
+    electronAPI?: ElectronAPI;
+  }
+}
+
+export {};