新增 token 余额前置检查，余额不足时阻止使用功能

问题：用户 token 余额为 0 时仍能正常使用文字聊天、语音聊天、实时
通话。根因是前端没有任何前置检查，完全依赖后端"先用后扣"模式，而
后端扣费失败仅记日志不阻断响应。

修复措施：
- 后端新增 GET /user/balance 轻量接口，返回 available_tokens
- 前端 chatStore 新增 fetchUserBalance()，页面初始化时查询余额并
  设置 _tokenExhausted / _realtimeTokenExhausted 标记
- sendTextMessage、toggleCallMode、toggleVoiceInput 入口处增加
  isTokenExhausted() 前置检查
- canSend 计算属性加入 token 检查，余额不足时发送按钮置灰

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/api/user_router.py

@@ -211,6 +211,23 @@ async def change_password(
         }
 
 
+@user_router.get("/balance")
+async def get_user_balance(
+    current_user: Dict = Depends(get_current_user),
+    db = Depends(get_db)
+):
+    """Get user available token balance"""
+    from services.token_service import TokenService
+    token_service = TokenService(db)
+    quota = await token_service._check_user_quota(current_user.get('id'))
+    return {
+        "success": True,
+        "data": {
+            "available_tokens": quota["available_tokens"]
+        }
+    }
+
+
 @user_router.get("/stats")
 async def get_user_stats(
     current_user: Dict = Depends(get_current_user),

frontend/src/api/index.js

@@ -76,7 +76,8 @@ export const userAPI = {
   login: (loginData) => apiClient.post(API_ENDPOINTS.USER.LOGIN, loginData),
   getProfile: () => apiClient.get(API_ENDPOINTS.USER.PROFILE),
   update: (data) => apiClient.put(API_ENDPOINTS.USER.UPDATE, data),
-  delete: () => apiClient.delete(API_ENDPOINTS.USER.PROFILE)
+  delete: () => apiClient.delete(API_ENDPOINTS.USER.PROFILE),
+  getBalance: () => apiClient.get('/api/v1/user/balance')
 }
 
 export const authAPI = userAPI

frontend/src/stores/chatStore.js

@@ -5,7 +5,7 @@
 import { defineStore } from 'pinia'
 import { ref, computed } from 'vue'
 import apiClient from '@/api/index'
-import { chatAPI } from '@/api/index'
+import { chatAPI, userAPI } from '@/api/index'
 import { ElMessage } from 'element-plus'
 
 // 统计更新事件回调
@@ -1744,6 +1744,19 @@ export const useChatStore = defineStore('chat', () => {
     _realtimeTokenExhausted = false
   }
 
+  async function fetchUserBalance() {
+    try {
+      const response = await userAPI.getBalance()
+      const available = response.data?.data?.available_tokens ?? 0
+      _tokenExhausted = available <= 0
+      _realtimeTokenExhausted = available <= 0
+      return available
+    } catch (error) {
+      console.warn('[ChatStore] Failed to fetch user balance:', error)
+      return null
+    }
+  }
+
   function updateSessionConfig(config) {
     currentSessionConfig.value = {
       ...currentSessionConfig.value,
@@ -1814,6 +1827,7 @@ export const useChatStore = defineStore('chat', () => {
     updateSessionConfig,
     isTokenExhausted,
     resetTokenExhausted,
+    fetchUserBalance,
 
     // 其他
     clearMessages,

frontend/src/views/ChatView.vue

@@ -1153,7 +1153,7 @@ export default {
     }
 
     const canSend = computed(() => {
-      return inputMessage.value.trim() && !sending.value && !isRecording.value && !isInCall.value
+      return inputMessage.value.trim() && !sending.value && !isRecording.value && !isInCall.value && !chatStore.isTokenExhausted()
     })
 
     const normalizeDateValue = (value) => {
@@ -1315,6 +1315,7 @@ export default {
         cacheSelectedPet(currentPet.value)
         saveChatConfig()
         syncStoreState()
+        await chatStore.fetchUserBalance()
         // ElMessage.success('聊天初始化成功')
       } catch (err) {
         console.error('Failed to init chat:', err)
@@ -1604,6 +1605,10 @@ export default {
 
     const sendTextMessage = async () => {
       if (!inputMessage.value.trim() || sending.value) return
+      if (chatStore.isTokenExhausted()) {
+        ElMessage.error('Token已耗尽，请充值后再使用')
+        return
+      }
 
       const messageText = inputMessage.value.trim()
       inputMessage.value = ''
@@ -1693,6 +1698,10 @@ export default {
         try {
           const hasPermission = await chatStore.requestMicrophonePermission()
           if (hasPermission) {
+            if (chatStore.isTokenExhausted()) {
+              ElMessage.error('Token已耗尽，请充值后再使用')
+              return
+            }
             await forceStopVoiceAndCall()
             // 先显示面板但不切换 chatMode，避免预连接期间的竞态
             showInputPanel.value = true
@@ -1826,6 +1835,11 @@ export default {
 
       pendingCallMode = true
       try {
+        if (chatStore.isTokenExhausted()) {
+          ElMessage.error('Token已耗尽，请充值后再使用')
+          return
+        }
+
         const hasPermission = await chatStore.requestMicrophonePermission()
         if (!hasPermission) {
           return