fix: 图片/文件读取改用 base64，修复 atob Latin1 错误

- 主进程新增 fs:readFileBase64 IPC，读 Buffer 直接返回 base64
- 图片读取从 atob 转换改为直接使用 readFileBase64
- .metona 备份导入同理修复
- 根因：fs:readFile 用 utf-8 编码读取二进制文件，UTF-8 多字节序列超出 Latin1 范围导致 atob 失败

src/renderer/types.d.ts

@@ -211,6 +211,7 @@ export interface MetonaDesktopAPI {
   };
   fs: {
     readFile: (filePath: string) => Promise<{ success: boolean; content?: string; name?: string; error?: string }>;
+    readFileBase64: (filePath: string) => Promise<{ success: boolean; content?: string; size?: number; name?: string; error?: string }>;
     writeFile: (filePath: string, content: string, encoding?: string) => Promise<{ success: boolean; error?: string }>;
   };
   tool: {