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/hooks/useNativeFolderPicker.ts

@@ -0,0 +1,20 @@
+import { useCallback } from 'react';
+
+/**
+ * Hook that provides native folder picker functionality in Electron,
+ * with detection for browser fallback.
+ */
+export function useNativeFolderPicker() {
+  const isElectron = typeof window !== 'undefined' && !!window.electronAPI?.dialog?.openFolder;
+
+  const openNativePicker = useCallback(
+    async (options?: { defaultPath?: string; title?: string }): Promise<string | null> => {
+      if (!window.electronAPI?.dialog?.openFolder) return null;
+      const result = await window.electronAPI.dialog.openFolder(options);
+      return result.canceled ? null : result.filePaths[0] ?? null;
+    },
+    []
+  );
+
+  return { isElectron, openNativePicker };
+}