🐛 fix: 统一充值页面路径为 /exchange

- 修复 redirectToPricePage 跳转路径不一致问题
- 统一使用 /exchange 作为充值页面入口

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

lib/api/balance.ts

@@ -1,5 +1,5 @@
-import { Alert } from 'react-native';
 import { router } from 'expo-router';
+import { Alert } from 'react-native';
 import { authClient } from '../auth/client';
 
 export interface UserBalance {
@@ -156,7 +156,7 @@ export async function recordTokenUsage(
           {
             text: '去充值',
             onPress: () => {
-              router.push('/(tabs)' as any);
+              router.push('/exchange');
             },
           },
         ]
@@ -210,5 +210,5 @@ export async function recordTokenUsage(
  * 跳转到充值页面
  */
 export function redirectToPricePage() {
-  router.push('/(tabs)' as any);
+  router.push('/exchange' as any);
 }