feat: 引入路由常量并更新相关页面导航

- 新增 ROUTES 常量文件，集中管理应用路由
- 更新多个页面的导航逻辑，使用 ROUTES 常量替代硬编码路径
- 修改教练页面和今日训练页面的路由，提升代码可维护性
- 优化标签页和登录页面的导航，确保一致性和易用性

app/(tabs)/_layout.tsx

@@ -7,6 +7,7 @@ import { IconSymbol } from '@/components/ui/IconSymbol';
 import { Colors } from '@/constants/Colors';
 import { TAB_BAR_BOTTOM_OFFSET, TAB_BAR_HEIGHT } from '@/constants/TabBar';
 import { useColorScheme } from '@/hooks/useColorScheme';
+import { ROUTES } from '@/constants/Routes';
 
 export default function TabLayout() {
   const theme = (useColorScheme() ?? 'light') as 'light' | 'dark';
@@ -17,9 +18,9 @@ export default function TabLayout() {
     <Tabs
       screenOptions={({ route }) => {
         const routeName = route.name;
-        const isSelected = (routeName === 'index' && pathname === '/') ||
-          (routeName === 'coach' && pathname === '/coach') ||
-          (routeName === 'statistics' && pathname === '/statistics') ||
+        const isSelected = (routeName === 'index' && pathname === ROUTES.TAB_HOME) ||
+          (routeName === 'coach' && pathname === ROUTES.TAB_COACH) ||
+          (routeName === 'statistics' && pathname === ROUTES.TAB_STATISTICS) ||
           pathname.includes(routeName);
 
         return {
@@ -39,7 +40,7 @@ export default function TabLayout() {
             const getIconAndTitle = () => {
               switch (routeName) {
                 case 'index':
-                  return { icon: 'house.fill', title: '首页' } as const;
+                  return { icon: 'magnifyingglass.circle.fill', title: '发现' } as const;
                 case 'coach':
                   return { icon: 'person.3.fill', title: 'Bot' } as const;
                 case 'statistics':
@@ -157,12 +158,12 @@ export default function TabLayout() {
       <Tabs.Screen
         name="index"
         options={{
-          title: '首页',
+          title: '发现',
           tabBarIcon: ({ color }) => {
             const isHomeSelected = pathname === '/' || pathname === '/index';
             return (
               <View style={{ flexDirection: 'row', alignItems: 'center' }}>
-                <IconSymbol size={22} name="house.fill" color={color} />
+                <IconSymbol size={22} name="magnifyingglass.circle.fill" color={color} />
                 {isHomeSelected && (
                   <Text
                     numberOfLines={1}
@@ -174,7 +175,7 @@ export default function TabLayout() {
                       textAlign: 'center',
                       flexShrink: 0,
                     }}>
-                    首页
+                    发现
                   </Text>
                 )}
               </View>