feat: 更新标签页和新增探索页面

- 将标签页中的“首页”改为“探索”，并更新相关逻辑
- 新增探索页面，展示推荐文章和训练计划
- 优化教练页面的导航，确保用户体验一致性
- 移除不再使用的代码，简化项目结构
- 更新路由常量，确保路径管理集中化

app/(tabs)/_layout.tsx

@@ -19,7 +19,7 @@ export default function TabLayout() {
       initialRouteName="coach"
       screenOptions={({ route }) => {
         const routeName = route.name;
-        const isSelected = (routeName === 'index' && pathname === ROUTES.TAB_HOME) ||
+        const isSelected = (routeName === 'explore' && pathname === ROUTES.TAB_EXPLORE) ||
           (routeName === 'coach' && pathname === ROUTES.TAB_COACH) ||
           (routeName === 'statistics' && pathname === ROUTES.TAB_STATISTICS) ||
           pathname.includes(routeName);
@@ -40,7 +40,7 @@ export default function TabLayout() {
             // 基于 routeName 设置图标与标题，避免 tabBarIcon 的包装导致文字裁剪
             const getIconAndTitle = () => {
               switch (routeName) {
-                case 'index':
+                case 'explore':
                   return { icon: 'magnifyingglass.circle.fill', title: '发现' } as const;
                 case 'coach':
                   return { icon: 'person.3.fill', title: 'Bot' } as const;
@@ -157,7 +157,7 @@ export default function TabLayout() {
         }}
       />
       <Tabs.Screen
-        name="index"
+        name="explore"
         options={{
           title: '发现',
           tabBarIcon: ({ color }) => {