feat(challenges): 新增挑战模块与详情页，优化标签栏布局

- 新增挑战列表页 `app/(tabs)/challenges.tsx`，展示热门挑战卡片
- 新增挑战详情页 `app/challenges/[id].tsx`，支持排行榜、分享与参与
- 在标签栏中新增“挑战”入口，替换原有“发现”与“AI”页
- 调整标签栏间距与圆角，适配新布局
- 新增挑战相关路由常量 `TAB_CHALLENGES`
- 迁移 `coach.tsx` 与 `explore.tsx` 至根目录，保持结构清晰

app/(tabs)/_layout.tsx

@@ -21,8 +21,8 @@ type TabConfig = {
 
 const TAB_CONFIGS: Record<string, TabConfig> = {
   statistics: { icon: 'chart.pie.fill', title: '健康' },
-  // explore: { icon: 'magnifyingglass.circle.fill', title: '发现' },
   goals: { icon: 'flag.fill', title: '习惯' },
+  challenges: { icon: 'trophy.fill', title: '挑战' },
   personal: { icon: 'person.fill', title: '个人' },
 };
 
@@ -35,9 +35,10 @@ export default function TabLayout() {
   // Helper function to determine if a tab is selected
   const isTabSelected = (routeName: string): boolean => {
     const routeMap: Record<string, string> = {
-      explore: ROUTES.TAB_EXPLORE,
-      goals: ROUTES.TAB_GOALS,
       statistics: ROUTES.TAB_STATISTICS,
+      goals: ROUTES.TAB_GOALS,
+      challenges: ROUTES.TAB_CHALLENGES,
+      personal: ROUTES.TAB_PERSONAL,
     };
 
     return routeMap[routeName] === pathname || pathname.includes(routeName);
@@ -69,11 +70,11 @@ export default function TabLayout() {
           alignItems: 'center',
           justifyContent: 'center',
           flexDirection: 'row',
-          marginHorizontal: 6,
+          marginHorizontal: 2,
           marginVertical: 10,
           borderRadius: 25,
           backgroundColor: isSelected ? colorTokens.tabBarActiveBackground : 'transparent',
-          paddingHorizontal: isSelected ? 16 : 10,
+          paddingHorizontal: isSelected ? 8 : 4,
           paddingVertical: 8,
         }}
       >
@@ -91,7 +92,7 @@ export default function TabLayout() {
                 fontWeight: '600',
                 marginLeft: 6,
               }}
-              numberOfLines={0 as any}
+              numberOfLines={1}
             >
               {tabConfig.title}
             </Text>
@@ -148,12 +149,12 @@ export default function TabLayout() {
       shadowOpacity: glassEffectAvailable ? 0.1 : 0.2,
       shadowRadius: 10,
       elevation: 5,
-      paddingHorizontal: 10,
+      paddingHorizontal: 6,
       paddingTop: 0,
       paddingBottom: 0,
-      marginHorizontal: 20,
-      left: 20,
-      right: 20,
+      marginHorizontal: 16,
+      left: 16,
+      right: 16,
       alignSelf: 'center',
       borderWidth: glassEffectAvailable ? 1 : 0,
       borderColor: glassEffectAvailable ? (theme === 'dark' ? 'rgba(255,255,255,0.1)' : 'rgba(0,0,0,0.1)') : 'transparent',
@@ -177,7 +178,11 @@ export default function TabLayout() {
       </NativeTabs.Trigger>
       <NativeTabs.Trigger name="goals">
         <Icon sf="flag.fill" drawable="custom_settings_drawable" />
-        <Label>目标</Label>
+        <Label>习惯</Label>
+      </NativeTabs.Trigger>
+      <NativeTabs.Trigger name="challenges">
+        <Icon sf="trophy.fill" drawable="custom_android_drawable" />
+        <Label>挑战</Label>
       </NativeTabs.Trigger>
       <NativeTabs.Trigger name="personal">
         <Icon sf="person.fill" drawable="custom_settings_drawable" />
@@ -193,9 +198,8 @@ export default function TabLayout() {
     >
 
       <Tabs.Screen name="statistics" options={{ title: '健康' }} />
-      <Tabs.Screen name="explore" options={{ title: '发现', href: null }} />
-      <Tabs.Screen name="coach" options={{ title: 'AI', href: null }} />
       <Tabs.Screen name="goals" options={{ title: '习惯' }} />
+      <Tabs.Screen name="challenges" options={{ title: '挑战' }} />
       <Tabs.Screen name="personal" options={{ title: '个人' }} />
     </Tabs>
   );