feat: add balance display, user columns, device mgmt and sessions for agent

- Backend: add balance to agent stats API
- Backend: return full user details (balance, expiry, online_status, device_count, application) in agent users API
- Backend: export admin device/session handlers, add agent routes for devices and sessions
- Backend: add card type permission check and balance deduction for agent user creation
- Backend: extract checkAgentUserPermission helper, add user edit API
- Frontend: add balance card to agent dashboard
- Frontend: add application, online_status, device_count, balance, expiry_at columns to agent user table
- Frontend: add agent devices page (reuse admin data-table component)
- Frontend: add agent sessions page (reuse admin data-table component)
- Frontend: add devices and sessions to agent navigation and routes
- Add i18n keys for agent devices, sessions, user columns, dashboard balance

frontend/src/plugins/i18n/en.json

@@ -2712,6 +2712,8 @@
       "totalUsersDesc": "Cumulative registered users",
       "totalRevenue": "Total Revenue",
       "todayRevenue": "Today",
+      "balance": "Account Balance",
+      "balanceDesc": "Available for card generation and recharge",
       "quickActions": "Quick Actions",
       "generateCards": "Generate Cards",
       "appManagement": "App Management",
@@ -2779,10 +2781,25 @@
       "columns": {
         "username": "Username",
         "email": "Email",
+        "application": "Application",
+        "onlineStatus": "Online Status",
+        "deviceCount": "Devices",
+        "balance": "Balance",
+        "expiryAt": "Expiry",
         "status": "Status",
         "createdAt": "Created At",
         "lastLoginAt": "Last Login"
       },
+      "onlineStatus": {
+        "online": "Online",
+        "offline": "Offline",
+        "banned": "Banned"
+      },
+      "expiry": {
+        "permanent": "Permanent",
+        "expired": "Expired",
+        "daysLeft": "days left"
+      },
       "status": {
         "active": "Active",
         "disabled": "Disabled",
@@ -2950,6 +2967,39 @@
         "generateNow": "Generate Now"
       }
     },
+    "devices": {
+      "title": "Device Management",
+      "description": "Manage devices under your applications",
+      "totalDevices": "Total Devices",
+      "onlineSessions": "Online Sessions",
+      "normalDevices": "Normal Devices",
+      "banned": "Banned",
+      "normal": "Normal",
+      "application": "Application",
+      "status": "Status",
+      "statusUpdateSuccess": "Device status updated successfully",
+      "statusUpdateFailed": "Failed to update device status",
+      "forceOffline": "Force Offline",
+      "forceOfflineTitle": "Force Offline",
+      "forceOfflineConfirm": "Are you sure you want to force offline all sessions for device \"{deviceId}\"?",
+      "forceOfflineSuccess": "Forced {count} sessions offline",
+      "forceOfflineFailed": "Failed to force offline"
+    },
+    "sessions": {
+      "title": "Online Sessions",
+      "description": "View all online session details",
+      "onlineSessions": "Online Sessions",
+      "onlineApps": "Online Apps",
+      "onlineDevices": "Online Devices",
+      "onlineUsers": "Online Users",
+      "application": "Application",
+      "loadFailed": "Failed to load sessions",
+      "forceOffline": "Force Offline",
+      "forceOfflineTitle": "Force Offline",
+      "forceOfflineConfirm": "Are you sure you want to force offline session \"{instanceId}\"?",
+      "forceOfflineSuccess": "Session forced offline successfully",
+      "forceOfflineFailed": "Failed to force offline session"
+    },
     "cloudVariables": {
       "title": "Cloud Variables",
       "description": "View cloud variables (read-only)",