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/preload/index.d.ts

@@ -316,6 +316,7 @@ declare global {
         httpStatus?: number
       }>
       startExport: (request: ExportRequest) => Promise<ExportResult>
+      selectExportDirectory: () => Promise<{ canceled: boolean; path?: string }>
       cancelExport: (jobId: string) => Promise<{ success: boolean }>
       revealExport: (path: string) => Promise<{ success: boolean; error?: string }>
       onExportProgress: (callback: (progress: ExportJobProgress) => void) => () => void

src/preload/index.ts

@@ -209,6 +209,7 @@ const api = {
   startExport: (request: ExportRequest) => ipcRenderer.invoke('export:start', request),
   cancelExport: (jobId: string) => ipcRenderer.invoke('export:cancel', jobId),
   revealExport: (path: string) => ipcRenderer.invoke('export:reveal', path),
+  selectExportDirectory: () => ipcRenderer.invoke('export:selectDirectory'),
   onExportProgress: (callback: (progress: ExportJobProgress) => void) => {
     const listener = (_event: Electron.IpcRendererEvent, progress: ExportJobProgress): void =>
       callback(progress)