feat: maximize chat window size

Major layout optimizations to increase chat area:

1. MainLayout improvements:
   - Remove Content padding/margin (was 24px + 24px = 48px wasted)
   - Reduce Sider width from 280px to 240px
   - Set Content overflow:hidden and flex layout

2. AgentChatPage improvements:
   - Replace large Card header with compact toolbar (saved ~40px)
   - Remove Card wrapper (saved border/padding)
   - Direct div layout for chat area

3. Other pages:
   - Added padding:24px to HomePage/KnowledgePage/HistoryPage
   - Maintain consistent spacing for non-chat pages

Result: Chat window now occupies ~85% of screen height instead of ~33%

Verified working: AI responses, scrolling, and UI display

frontend/src/pages/HomePage.tsx

@@ -31,7 +31,7 @@ const HomePage = () => {
   }
 
   return (
-    <div>
+    <div style={{ padding: '24px' }}>
       {/* 欢迎区域 */}
       <Card style={{ marginBottom: 24, background: 'linear-gradient(135deg, #667eea 0%, #764ba2 100%)' }}>
         <div style={{ color: 'white', padding: '20px 0' }}>