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

admin-frontend/src/router/index.js

@@ -44,6 +44,14 @@ const routes = [
       { path: '', name: 'Payments', component: () => import('@/views/Payments.vue'), meta: { title: '支付管理' } },
     ]
   },
+  {
+    path: '/credits',
+    component: AdminLayout,
+    meta: { requiresAuth: true },
+    children: [
+      { path: '', name: 'Credits', component: () => import('@/views/CreditManagement.vue'), meta: { title: '信用管理' } },
+    ]
+  },
   {
     path: '/config',
     component: AdminLayout,