feat: 前台侧边栏改为分组导航，微信机器人归入 IM Bot 分组

frontend/src/components/layout/FrontLayout.css

@@ -80,6 +80,14 @@
   flex-direction: column;
   gap: 2px;
 }
+.nav-group-title {
+  padding: 14px var(--space-md) 6px;
+  font-size: 11px;
+  font-weight: 600;
+  letter-spacing: 0.06em;
+  text-transform: uppercase;
+  color: #9CA3AF;
+}
 
 .nav-item {
   position: relative;

frontend/src/components/layout/FrontLayout.tsx

@@ -3,16 +3,31 @@ import { Outlet, NavLink, useNavigate } from 'react-router-dom';
 import { useAuth } from '@/contexts/AuthContext';
 import './FrontLayout.css';
 
-const navItems = [
+const navGroups = [
+  {
+    title: '智能体',
+    items: [
       { path: '/chat', icon: 'chat', label: 'AI聊天' },
-  { path: '/channels', icon: 'channel', label: 'IM Bot' },
       { path: '/knowledge', icon: 'knowledge', label: '知识库' },
       { path: '/market', icon: 'market', label: '技能市场' },
+      { path: '/evolution', icon: 'evolution', label: 'AI自进化' },
+    ],
+  },
+  {
+    title: 'IM Bot',
+    items: [
+      { path: '/channels', icon: 'channel', label: '频道管理' },
+      { path: '/wechat-bot', icon: 'wechat', label: '微信机器人' },
+    ],
+  },
+  {
+    title: '资源中心',
+    items: [
+      { path: '/assets', icon: 'assets', label: '资产文件' },
       { path: '/token', icon: 'token', label: 'Token驾驶舱' },
       { path: '/pricing', icon: 'pricing', label: '付费中心' },
-  { path: '/assets', icon: 'assets', label: '资产文件' },
-  { path: '/evolution', icon: 'evolution', label: 'AI自进化' },
-  { path: '/wechat-bot', icon: 'wechat', label: '微信机器人' },
+    ],
+  },
 ];
 
 const bottomItems = [
@@ -80,8 +95,10 @@ const FrontLayout: React.FC = () => {
             </span>
             {!collapsed && <span className="nav-item-label">返回首页</span>}
           </NavLink>
-          <div className="nav-section">
-            {navItems.map(item => (
+          {navGroups.map(group => (
+            <div className="nav-section" key={group.title}>
+              {!collapsed && <div className="nav-group-title">{group.title}</div>}
+              {group.items.map(item => (
                 <NavLink
                   key={item.path}
                   to={item.path}
@@ -94,6 +111,7 @@ const FrontLayout: React.FC = () => {
                 </NavLink>
               ))}
             </div>
+          ))}
           <div className="nav-section nav-bottom">
             {bottomItems.map(item => (
               <NavLink