fix: 优化 Windows 启动性能与聊天图片缓存

- 增加聊天图片磁盘持久化缓存，重启后直接复用
- 将 DAT 图片解密移至 Worker，避免阻塞主进程
- 增加图片加载优先级和并发控制
- 优化会话目录与图片文件的异步查找
- 使用本地媒体协议加载缓存图片，减少 Base64 IPC 开销
- 优化启动缓存与数据库初始化流程，降低窗口未响应时间

(cherry picked from commit 82bcc8d32ca674de38c745cc9925ed2309887dc7)

src/preload/index.d.ts

@@ -205,7 +205,7 @@ declare global {
         imageMd5?: string,
         imageDatNameOrThumb?: string | boolean,
         sessionId?: string,
-        options?: { force?: boolean; preferThumbnail?: boolean }
+        options?: { force?: boolean; preferThumbnail?: boolean; priority?: number }
       ) => Promise<{
         success: boolean
         data?: string

src/preload/index.ts

@@ -76,7 +76,7 @@ const api = {
     imageMd5?: string,
     imageDatNameOrThumb?: string | boolean,
     sessionId?: string,
-    options?: { force?: boolean; preferThumbnail?: boolean }
+    options?: { force?: boolean; preferThumbnail?: boolean; priority?: number }
   ) => ipcRenderer.invoke('db:getImage', imageMd5, imageDatNameOrThumb, sessionId, options),
   getVideo: (hashes: string[]) => ipcRenderer.invoke('db:getVideo', hashes),
   getSticker: (cdnUrl?: string, md5?: string) => ipcRenderer.invoke('db:getSticker', cdnUrl, md5),