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/components/chat/MessageList.css

@@ -1,15 +1,21 @@
 .message-list {
   flex: 1;
   overflow-y: auto;
-  padding: 24px;
-  background: #f5f5f5;
+  overflow-x: hidden;
+  padding: 20px 16px 20px 20px; /* 右侧留空给滚动条 */
+  background: linear-gradient(180deg, #f5f7fa 0%, #fafbfc 100%);
+  min-height: 0; /* 重要：让flex容器可以正确计算滚动 */
+  max-height: 100%; /* 限制最大高度 */
+  position: relative;
+  scroll-behavior: smooth; /* 平滑滚动 */
 }
 
 .message-item {
   display: flex;
   gap: 12px;
-  margin-bottom: 24px;
+  margin-bottom: 20px;
   animation: fadeIn 0.3s ease-in;
+  max-width: 100%;
 }
 
 @keyframes fadeIn {
@@ -40,14 +46,31 @@
 
 .message-body {
   background: white;
-  border-radius: 8px;
-  padding: 12px 16px;
-  box-shadow: 0 1px 2px rgba(0, 0, 0, 0.05);
+  border-radius: 12px;
+  padding: 14px 18px;
+  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.08);
+  max-width: 85%;
+  word-wrap: break-word;
+  transition: box-shadow 0.2s ease;
+}
+
+.message-body:hover {
+  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.12);
 }
 
 .message-user .message-body {
-  background: #e6f7ff;
-  border: 1px solid #91d5ff;
+  background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
+  border: none;
+  color: white;
+}
+
+.message-user .user-message-text {
+  color: white;
+}
+
+.message-assistant .message-body {
+  background: white;
+  border: 1px solid #e8e8e8;
 }
 
 .user-message-text {
@@ -168,22 +191,22 @@
   border-top: 1px solid #f0f0f0;
 }
 
-/* 滚动条样式 */
+/* 滚动条样式 - 优化 */
 .message-list::-webkit-scrollbar {
-  width: 8px;
+  width: 6px;
 }
 
 .message-list::-webkit-scrollbar-track {
-  background: #f1f1f1;
-  border-radius: 4px;
+  background: transparent;
 }
 
 .message-list::-webkit-scrollbar-thumb {
-  background: #888;
-  border-radius: 4px;
+  background: rgba(0, 0, 0, 0.2);
+  border-radius: 3px;
+  transition: background 0.2s;
 }
 
 .message-list::-webkit-scrollbar-thumb:hover {
-  background: #555;
+  background: rgba(0, 0, 0, 0.35);
 }