fix(dc-tool-b): 修复滚动条问题 - 适配MainLayout固定高度

- tool-b/index.tsx: min-h-screen  h-full overflow-y-auto
- 所有Step组件添加 h-full overflow-y-auto
- 修复：MainLayout改为固定高度后，工具B底部按钮可见性问题
- 效果：页面无滚动条，内容可内部滚动

frontend-v2/src/modules/dc/pages/tool-b/Step3Processing.tsx

@@ -108,7 +108,7 @@ const Step3Processing: React.FC<Step3ProcessingProps> = ({ state, updateState, o
   }, []);
 
   return (
-    <div className="max-w-4xl mx-auto w-full space-y-6 animate-in fade-in slide-in-from-bottom-4 duration-500 mt-8">
+    <div className="max-w-4xl mx-auto w-full space-y-6 animate-in fade-in slide-in-from-bottom-4 duration-500 mt-8 h-full overflow-y-auto">
       <div className="text-center">
         <h3 className="text-xl font-bold text-slate-900 mb-2">双模型提取交叉验证中...</h3>
         <p className="text-sm text-slate-500">DeepSeek-V3 & Qwen-Max 双引擎协同工作</p>