feat(auth): add device session tracking with configurable limits

Track authenticated sessions per device (parsing user-agent for desktop
vs mobile via mileusna/useragent), enforce configurable concurrent device
limits (Auth.DeviceLimits), and surface device status and "remove other
devices" management in the account dialog. Adds device/session context to
the auth module stores and exposes limits through the API.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/i18n/locales/en-US.json

@@ -19,16 +19,27 @@
   "username": "Username",
   "email": "Email",
   "deviceManagement": "Device Management",
-  "removeAllDevices": "Remove All Devices",
-  "removeAllDevicesDescription": "A Moteva account is for personal use only. To prevent abuse and keep the platform stable, tasks can run in up to 2 desktop web sessions and 1 mobile web session at the same time. For team or multi-person use, please purchase Team Plan.",
+  "removeAllDevices": "Remove Other Devices",
+  "removeAllDevicesDescription": "A Moteva account is for personal use only. To prevent abuse and keep the platform stable, tasks can run in up to {desktop} desktop web sessions and {mobile} mobile web sessions at the same time. For team or multi-person use, please purchase Team Plan.",
+  "deviceLimitsLoading": "Loading the current device policy.",
+  "deviceLimitsUnavailable": "The current device policy is temporarily unavailable.",
   "onlineStatus": "Online",
   "deviceInfo": "Device Info",
+  "deviceStatus": "Device status",
   "currentDevice": "Current Device",
   "system": "System",
   "currentLoginAccount": "Current account: {name}",
   "accountDeviceDesktop": "Desktop",
+  "accountDeviceMobile": "Mobile",
   "accountDeviceUnknownSystem": "Unknown system",
   "accountDeviceUnknownBrowser": "Unknown browser",
+  "deviceOnline": "Online",
+  "deviceOffline": "Offline",
+  "deviceLastSeen": "Last active {time}",
+  "deviceListLoading": "Loading devices",
+  "deviceListLoadFailed": "Could not load devices",
+  "noActiveDevices": "No active devices",
+  "retry": "Retry",
   "avatarImageRequired": "Please choose an image file.",
   "avatarTooLarge": "Avatar image must be under 2 MB.",
   "avatarUpdateFailed": "Failed to update avatar. Please choose the image again.",
@@ -37,9 +48,11 @@
   "usernameUpdated": "Username updated.",
   "usernameUpdateFailed": "Failed to update username. Please try again later.",
   "currentDeviceNotice": "This is the device you are using now.",
-  "devicesRemoved": "{count} device(s) removed.",
-  "removeAllDevicesConfirmTitle": "Remove all devices?",
-  "removeAllDevicesConfirmText": "After confirmation, your current session will be removed and you will need to log in again.",
+  "devicesRemoved": "{count} other device(s) removed.",
+  "devicesRemoving": "Removing",
+  "devicesRemoveFailed": "Could not remove the other devices. Please try again.",
+  "removeAllDevicesConfirmTitle": "Remove other devices?",
+  "removeAllDevicesConfirmText": "This will sign out {count} other device(s). Your current device will stay signed in.",
   "guides": "Guides",
   "contactUs": "Contact us",
   "logout": "Log out",