强化分割结果导出按钮样式

- 为工作区分割结果导出按钮增加 FileDown 图标，让导出入口更容易识别。

- 将导出按钮从灰色弱背景调整为绿色强调背景、边框和文字样式，提升在顶栏中的区分度。

- 补充 VideoWorkspace 测试，覆盖导出按钮图标和强调背景样式。

- 更新前端元素审计、测试计划和 AGENTS 文档，记录导出按钮视觉规范。

src/components/VideoWorkspace.tsx

@@ -1,5 +1,5 @@
 import React, { useCallback, useEffect, useMemo, useState } from 'react';
-import { Redo, Undo } from 'lucide-react';
+import { FileDown, Redo, Undo } from 'lucide-react';
 import { useStore } from '../store/useStore';
 import {
   annotationToMask,
@@ -2096,9 +2096,10 @@ export function VideoWorkspace({ onNavigateToAI }: { onNavigateToAI?: () => void
               disabled={!currentProject?.id || isExporting || isSaving || isPropagating}
               aria-haspopup="menu"
               aria-expanded={isExportMenuOpen}
-              className="px-4 py-1.5 bg-white/5 hover:bg-white/10 border border-white/10 rounded-md text-xs transition-colors text-white disabled:opacity-40 disabled:cursor-not-allowed"
+              className="inline-flex items-center gap-1.5 px-4 py-1.5 bg-emerald-500/20 hover:bg-emerald-500/30 border border-emerald-400/40 rounded-md text-xs font-medium transition-colors text-emerald-50 shadow-sm shadow-emerald-950/30 disabled:opacity-40 disabled:cursor-not-allowed"
             >
-              {isExporting ? '导出中...' : '分割结果导出'}
+              <FileDown aria-hidden="true" className="h-3.5 w-3.5 text-emerald-200" />
+              <span>{isExporting ? '导出中...' : '分割结果导出'}</span>
             </button>
             {isExportMenuOpen && (
               <div