Support Siri/CarPlay message announcements via INSendMessageIntent

**Is your feature request related to a problem? Please describe.**
We've recently rolled out Nextcloud Talk across our engineering team as our primary messaging platform. Several engineers spend a lot of time driving between sites and have asked whether Talk messages can be read aloud through CarPlay, the same way iMessage and WhatsApp messages are announced via Siri while driving. Currently Talk doesn't appear to integrate with this at all - no announcements, no CarPlay message list.

**Describe the solution you'd like**
Donate an INSendMessageIntent (with INSpeakableString for sender/content) to Siri when a chat notification is received, similar to how WhatsApp and Telegram support "Announce Messages." This would let users enable Settings -> Siri & Search -> Announce Notifications for Talk, and have Siri read incoming messages aloud through CarPlay/AirPods, with the option to reply by dictation.

This is distinct from the existing CallKit/CarPlay integration for calls (e.g. #743) - this request is specifically about chat message announcements, not call audio.

**Describe alternatives you've considered**
Standard push notifications work, but they don't get announced or read aloud while driving, so engineers have to glance at or pick up their phone to see who messaged and what it says - not ideal while driving.

**Additional context**
Rolled out for a refrigeration/HVAC engineering company where field engineers are frequently in vehicles between jobs; hands-free message awareness while driving is a genuine safety/usability need for us, not just a nice-to-have.