feat: 更新Chat前端页面

- 优化Chat组件交互体验
- 修复智能体选择逻辑
- 新增chat页面样式和脚本

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/components/chat/ChatInput.vue

@@ -41,7 +41,7 @@ const autoResize = (e: Event) => {
           @keydown="handleKeydown"
           placeholder="发送消息..."
           rows="1"
-          class="w-full bg-transparent text-white placeholder-white/25 py-4 pl-12 pr-28 resize-none focus:outline-none text-[15px]"
+          class="chat-input-textarea w-full bg-transparent text-white placeholder-white/25 py-4 pl-12 pr-28 resize-none focus:outline-none text-[15px]"
         ></textarea>
 
         <!-- 发送按钮 -->
@@ -63,9 +63,10 @@ const autoResize = (e: Event) => {
         </button>
       </div>
 
-      <!-- 提示 -->
-      <div class="text-center mt-3">
+      <!-- 字符计数 -->
+      <div class="flex justify-between items-center mt-2 px-1">
         <span class="text-[10px] text-white/20 tracking-wide">AI 可能会产生错误信息，请核实重要内容</span>
+        <span class="text-[10px] text-white/20">{{ modelValue.length }}/4000</span>
       </div>
     </div>
   </div>