[2.x] feat(realtime): make notification toast duration configurable

[imorland]

Summary

• Replaces the hardcoded 5s dismiss timeout with an admin setting flarum-realtime.notification-toast-dismiss-after, defaulting to 10 seconds
• Setting the value to 0 disables toast notifications entirely (the push() call returns early, nothing is queued or rendered)
• The value is registered as a PHP default, serialized to the forum payload, and picked up directly from app.data.settings in NotificationToastState
• Admin setting registered in admin/extend.ts with min: 0

Test plan

• Set duration to a custom value and confirm toasts dismiss after that many seconds
• Set duration to 0 and confirm no toasts appear for incoming realtime notifications