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/Profile.vue

@@ -11,8 +11,8 @@
           </div>
           <el-divider style="margin:8px 0" />
           <div class="profile-menu">
-            <div class="menu-item" @click="$router.push('/upgrade')">
-              <el-icon><Crown /></el-icon><span>升级会员</span>
+            <div class="menu-item" @click="$router.push('/credits')">
+              <el-icon><Coin /></el-icon><span>购买次数</span>
             </div>
             <div class="menu-item" @click="$router.push('/certification')">
               <el-icon><Stamp /></el-icon><span>实名认证</span>