Chat: offer to notify when a reply is ready, and say the page can be closed #695

Describe the feature you'd like to request

While a chat reply is generating there is no way to be told when it is done. Nothing in the UI says the generation keeps running on the server, so the page is kept open waiting, and long replies mean staring at the skeleton row.

Describe the solution you'd like

While a reply is generating, show a small banner above the input:

• one line telling the user the page can be closed
• a Notify toggle that opts into a notification for this reply

Opting in asks for browser notification permission and sends the task notification when the reply finishes (success or failure). Clicking the notification, its View reply action, or the list entry opens the conversation it belongs to.

Browser notifications for closed tabs need the background push click fix from nextcloud/notifications#3477, implemented in nextcloud/notifications#3478.

Describe alternatives you've considered

Mail notifications (slower, needs an address) and always-on notifications (too noisy for an opt-in-per-reply feature).