iOS: Support Apple’s Communication Notifications API for Focus mode contact exceptions

### Is your feature request related to a problem? Please describe.

Yes.

I use Nextcloud Talk as my primary messenger on iOS. When using Focus modes such as Sleep, Do Not Disturb or Work, I would like to receive messages only from selected people, just like I can with iMessage or WhatsApp.

Currently, Nextcloud Talk notifications are treated as generic app notifications by iOS. As a result, I can only allow or silence the entire app, but I cannot allow messages from specific contacts while keeping all other conversations muted. This makes it difficult to use Talk as a full replacement for other messaging apps.

### Describe the solution you'd like

Please support Apple's Communication Notifications framework (e.g. Communication Notifications / INSendMessageIntent) so that iOS recognizes Nextcloud Talk messages as person-to-person communication.

This would allow users to receive notifications from selected Talk contacts during Focus modes while keeping all other conversations and groups silent. It would also provide a more seamless integration with the iOS notification ecosystem and make Nextcloud Talk behave consistently with other modern messaging apps.

### Describe alternatives you've considered

The only current workaround is to allow the entire Nextcloud Talk app during Focus mode. However, this also allows notifications from every conversation and group, which defeats the purpose of Focus modes. There is currently no practical way to allow only important contacts.

### Additional context

Apps such as iMessage and WhatsApp already support this functionality on iOS. For users who rely on Nextcloud Talk as their primary messenger, this feature would significantly improve the overall user experience and make Talk a more compelling alternative to proprietary messaging platforms.

Thank you for considering this feature.

Start by reviewing the iOS notification handling and Apple’s Communication Notifications framework, especially INSendMessageIntent, as named in the issue. Trace how Talk messages currently become notifications. Done means iOS recognizes person-to-person Talk messages and Focus mode can allow selected contacts while muting other conversations and groups.

Written by the indexing model from the issue text.