修复bug，第一次导出缩略图，后面有了高清图后再次导出应该覆盖

按原图、中图和缩略图对本地图片资源分级，始终优先选择高清变体。

增量导出仅复用满足清晰度要求的图片，重新探测旧缩略图并在高清图出现后替换；仅缩略图降级时显示提示。

src/main/services/chat-service.ts

@@ -10,6 +10,7 @@ import type {
   DatabaseKeyValidationResult
 } from '../../shared/database-key'
 import { mergeRecallArchiveMessages, recordRecallArchiveMessages } from './recall-archive-service'
+import type { ExportImageQuality } from '../../shared/image-quality'
 
 export function getCurrentKey(): string {
   if (!dbRef) return ''
@@ -60,6 +61,7 @@ export interface FormattedMessage {
   exportMediaUrl?: string
   exportMediaType?: 'image' | 'video' | 'sticker' | 'file'
   exportMediaName?: string
+  exportMediaQuality?: ExportImageQuality
   exportShowAvatar?: boolean
   exportMediaError?: string
   exportAvatarUrl?: string