feat: frontend credit system UI

Admin:
- New CreditManagement.vue (tabs: rates, packages, plans, user credits, consumptions, stats)
- Sidebar menu + router entry
- Full CRUD for credit packages and subscription plans
- User credit balance adjustment
- Consumption log viewer

User:
- Credits.vue replaces Upgrade.vue (package purchase, subscription, history tabs)
- Credit balance display in topbar + dashboard header CTA card
- Navigation restructured: discovery first
- Profile redirects to /credits
- Dashboard upgrade dialog simplified to redirect to /credits

user-frontend/src/views/Workspace.vue

@@ -20,10 +20,24 @@
       style="margin-bottom:16px"
     >
       <template #default>
-        <span>试用结束后将自动恢复为免费版。 <el-button text type="primary" size="small" @click="showUpgrade = true">立即升级正式版</el-button></span>
+        <span>试用结束后将自动恢复为免费版。 <el-button text type="primary" size="small" @click="$router.push('/credits')">购买更多次数</el-button></span>
       </template>
     </el-alert>
 
+    <el-card shadow="never" style="margin-bottom:16px;background:linear-gradient(135deg,#667eea 0%,#764ba2 100%);color:#fff">
+      <div style="display:flex;align-items:center;justify-content:space-between;padding:8px 0">
+        <div>
+          <div style="font-size:14px;opacity:0.9">信用余额</div>
+          <div style="font-size:28px;font-weight:700">{{ creditBalance ?? '...' }} 次</div>
+        </div>
+        <div style="text-align:right">
+          <el-button type="primary" @click="$router.push('/discovery')" style="margin-bottom:4px">发现新客户</el-button>
+          <br>
+          <el-button text style="color:#fff;opacity:0.8" @click="$router.push('/credits')">购买次数 →</el-button>
+        </div>
+      </div>
+    </el-card>
+
     <el-row :gutter="20">
       <el-col :xs="12" :sm="6" v-for="item in stats" :key="item.label">
         <el-card shadow="hover" class="stat-card" @click="item.route && $router.push(item.route)">
@@ -37,7 +51,7 @@
       <template #header>
         <div class="card-header">
           <span class="section-title">本月用量</span>
-          <el-button v-if="usageStats.tier !== 'enterprise'" text type="primary" size="small" @click="showUpgrade = true">升级以获取更多额度</el-button>
+          <el-button v-if="usageStats.tier !== 'enterprise'" text type="primary" size="small" @click="$router.push('/credits')">购买更多次数</el-button>
         </div>
       </template>
       <div class="usage-grid">
@@ -139,7 +153,7 @@
         </el-table-column>
       </el-table>
       <div style="text-align:center;margin-top:20px">
-        <el-button type="primary" size="large" @click="$router.push('/upgrade')">立即升级</el-button>
+        <el-button type="primary" size="large" @click="$router.push('/credits')">购买次数</el-button>
       </div>
     </el-dialog>
   </div>
@@ -148,6 +162,7 @@
 <script setup>
 import { ref, computed, onMounted } from 'vue'
 import { useAuthStore } from '@/stores/auth'
+import { getCreditBalance } from '@/api'
 import { getAnalyticsOverview, translate, getFollowupPending, getSilentCustomers, getUsageStats } from '@/api'
 
 const auth = useAuthStore()
@@ -160,6 +175,7 @@ const followups = ref([])
 const silentCustomers = ref([])
 const usageStats = ref({ tier: 'free', limits: {}, usage: {} })
 const showUpgrade = ref(false)
+const creditBalance = ref(null)
 
 const usageItems = computed(() => {
   const u = usageStats.value.usage || {}
@@ -206,6 +222,7 @@ const features = [
 
 onMounted(async () => {
   try {
+    getCreditBalance().then(res => { creditBalance.value = res.balance }).catch(() => {})
     const [overview, fup, silent, usage] = await Promise.all([
       getAnalyticsOverview().catch(() => null),
       getFollowupPending().catch(() => []),