fix: 修复截图 MIME 类型硬编码导致 API 拒绝的问题

macOS screencapture 输出 PNG，但代码硬编码 mimeType 为 image/jpeg，
导致 API 报错 "specified using image/jpeg but appears to be image/png"。
改为通过 magic bytes 检测实际图片格式。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/utils/computerUse/wrapper.tsx

@@ -30,6 +30,7 @@ import { getSessionId } from '../../bootstrap/state.js'
 import { ComputerUseApproval } from '../../components/permissions/ComputerUseApproval/ComputerUseApproval.js'
 import type { Tool, ToolUseContext } from '../../Tool.js'
 import { logForDebugging } from '../debug.js'
+import { detectImageFormatFromBase64 } from '../imageResizer.js'
 import {
   checkComputerUseLock,
   tryAcquireComputerUseLock,
@@ -330,7 +331,7 @@ export function getComputerUseMCPToolOverrides(
                 type: 'image' as const,
                 source: {
                   type: 'base64' as const,
-                  media_type: item.mimeType ?? 'image/jpeg',
+                  media_type: item.mimeType ?? detectImageFormatFromBase64(item.data),
                   data: item.data,
                 },
               }