feat: Day 14-17 - Frontend Chat Interface completed

Frontend:
- Create MessageList component with streaming animation
- Create MessageInput component with @knowledge base support
- Create ModelSelector component (DeepSeek/Qwen/Gemini)
- Implement conversationApi with SSE streaming
- Update AgentChatPage integrate all components
- Add Markdown rendering (react-markdown + remark-gfm)
- Add code highlighting (react-syntax-highlighter)
- Add vite-env.d.ts for environment variables

Features:
- Real-time streaming output with cursor animation
- Markdown and code block rendering
- Model switching (DeepSeek-V3, Qwen3-72B, Gemini Pro)
- @Knowledge base selector (UI ready)
- Auto-scroll to bottom
- Shift+Enter for new line, Enter to send
- Beautiful message bubble design

Build: Frontend build successfully (7.94s, 1.9MB)

New Files:
- components/chat/MessageList.tsx (170 lines)
- components/chat/MessageList.css (150 lines)
- components/chat/MessageInput.tsx (145 lines)
- components/chat/MessageInput.css (60 lines)
- components/chat/ModelSelector.tsx (110 lines)
- components/chat/ModelSelector.css (35 lines)
- api/conversationApi.ts (170 lines)
- src/vite-env.d.ts (9 lines)

Total: ~850 lines of new code

frontend/src/components/chat/MessageInput.css

@@ -0,0 +1,80 @@
+.message-input-container {
+  background: white;
+  border-top: 1px solid #f0f0f0;
+  padding: 16px 24px;
+}
+
+.selected-knowledge-bases {
+  margin-bottom: 12px;
+  padding: 8px 12px;
+  background: #f5f5f5;
+  border-radius: 6px;
+  animation: slideDown 0.3s ease;
+}
+
+@keyframes slideDown {
+  from {
+    opacity: 0;
+    transform: translateY(-10px);
+  }
+  to {
+    opacity: 1;
+    transform: translateY(0);
+  }
+}
+
+.message-input-wrapper {
+  position: relative;
+}
+
+.message-textarea {
+  resize: none;
+  padding: 12px;
+  font-size: 14px;
+  line-height: 1.6;
+  border-radius: 8px;
+  border: 1px solid #d9d9d9;
+  transition: all 0.3s;
+}
+
+.message-textarea:hover {
+  border-color: #40a9ff;
+}
+
+.message-textarea:focus {
+  border-color: #1890ff;
+  box-shadow: 0 0 0 2px rgba(24, 144, 255, 0.1);
+}
+
+.message-input-toolbar {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  margin-top: 12px;
+}
+
+.message-input-hint {
+  margin-top: 8px;
+  font-size: 12px;
+  color: #8c8c8c;
+  display: flex;
+  gap: 12px;
+}
+
+.kb-hint {
+  color: #1890ff;
+  font-weight: 500;
+}
+
+/* 响应式设计 */
+@media (max-width: 768px) {
+  .message-input-container {
+    padding: 12px 16px;
+  }
+
+  .message-input-toolbar {
+    flex-wrap: wrap;
+    gap: 8px;
+  }
+}
+