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/main/ipc.ts

@@ -53,6 +53,15 @@ export function setupIPC(): void {
     }
   });
 
+  ipcMain.handle('fs:readFileBase64', async (_, filePath: string) => {
+    try {
+      const buf = await fs.promises.readFile(filePath);
+      return { success: true, content: buf.toString('base64'), size: buf.length, name: path.basename(filePath) };
+    } catch (err) {
+      return { success: false, error: (err as Error).message };
+    }
+  });
+
   ipcMain.handle('fs:writeFile', async (_, filePath: string, content: string, encoding?: string) => {
     try {
       await fs.promises.writeFile(filePath, content, (encoding as BufferEncoding) || 'utf-8');