feat: 支持导出自定义目录并优化聊天记录读取

接入导出保存目录选择功能
修复导出路径摘要未同步更新
按时间范围查询聊天记录，避免全量扫描历史消息
增加跨分片消息查询支持验证

src/main/index.ts

@@ -1164,6 +1164,11 @@ app.whenReady().then(async () => {
     if (!window) return { success: false, error: '窗口不可用' }
     return runExport(request, window, voiceRecognition || undefined)
   })
+  ipcMain.handle('export:selectDirectory', async (event) => {
+    const window = BrowserWindow.fromWebContents(event.sender)
+    const result = await dialog.showOpenDialog(window!, { properties: ['openDirectory', 'createDirectory'] })
+    return result.canceled ? { canceled: true } : { canceled: false, path: result.filePaths[0] }
+  })
   ipcMain.handle('export:cancel', (_, jobId: string) => {
     cancelExport(jobId)
     return { success: true }