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/input-area.ts

@@ -136,20 +136,19 @@ async function handleImagePaths(paths: string[]): Promise<void> {
   for (const filePath of paths) {
     const name = filePath.split(/[/\\]/).pop() || filePath;
     try {
-      const result = await bridge.fs.readFile(filePath);
+      const result = await bridge.fs.readFileBase64(filePath);
       if (!result.success) {
         appendSystemMessage(`❌ 读取 ${name} 失败: ${result.error}`);
         continue;
       }
-      // IPC 返回的 content 是 UTF-8 字符串，转为 base64
-      const binary = Uint8Array.from(atob(result.content as string), c => c.charCodeAt(0));
-      const base64 = btoa(String.fromCharCode(...binary));
-      if (binary.length > maxSize) {
+      const size = result.size ?? 0;
+      if (size > maxSize) {
         appendSystemMessage(`⚠️ 图片 ${name} 超过 20MB 限制`);
         continue;
       }
+      const base64 = result.content!;
       pendingImages.push({ name, base64 });
-      logInfo(`图片已添加: ${name}`, formatSize(binary.length));
+      logInfo(`图片已添加: ${name}`, formatSize(size));
     } catch (err) {
       logError('图片读取失败', (err as Error).message);
       appendSystemMessage(`❌ 读取 ${name} 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);