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/components/settings-modal.ts

@@ -298,12 +298,12 @@ async function importSessions(filePath: string): Promise<void> {
 
   try {
     const bridge = (window as any).metonaDesktop;
-    const result = await bridge.fs.readFile(filePath);
+    const result = await bridge.fs.readFileBase64(filePath);
     if (!result.success) {
       showToast(`读取文件失败: ${result.error}`, 'error');
       return;
     }
-    // IPC 返回 base64 编码的二进制，转 ArrayBuffer
+    // 主进程返回 base64，直接解码为二进制
     const binary = atob(result.content as string);
     const bytes = new Uint8Array(binary.length);
     for (let i = 0; i < binary.length; i++) bytes[i] = binary.charCodeAt(i);