chore(channels): drop admin-side available channels view

Remove the admin-side "Available Channels" aggregate view — admins
already see full channel configuration (groups, pricing, model
mappings) in the channel edit dialog, making a read-only admin
aggregate view redundant. The user-side "可用渠道" remains.

Backend:
- Delete handler/admin/available_channel_handler.go (+ test)
- Drop AdminHandlers.AvailableChannel field and wire injection
- Remove /admin/channels/available route

Frontend:
- Delete views/admin/AvailableChannelsView.vue
- Drop /admin/available-channels router entry
- Strip AvailableChannel types + listAvailable from api/admin/channels.ts

frontend/src/router/index.ts

@@ -370,18 +370,6 @@ const routes: RouteRecordRaw[] = [
       descriptionKey: 'admin.groups.description'
     }
   },
-  {
-    path: '/admin/available-channels',
-    name: 'AdminAvailableChannels',
-    component: () => import('@/views/admin/AvailableChannelsView.vue'),
-    meta: {
-      requiresAuth: true,
-      requiresAdmin: true,
-      title: 'Available Channels',
-      titleKey: 'admin.availableChannels.title',
-      descriptionKey: 'admin.availableChannels.description'
-    }
-  },
   {
     path: '/admin/channels',
     redirect: '/admin/channels/pricing'