Apple CarPlay support

## Feature request

**Is your feature request related to a problem? Please describe.**
Nextcloud Talk for iOS does not appear in the CarPlay app list under iPhone Settings → General → CarPlay. This means it is completely absent from the CarPlay interface. Users who drive regularly and need to stay connected with their team through Nextcloud Talk are forced to pick up their phone to read messages or make calls, which is both dangerous and illegal in many countries.

**Describe the solution you'd like**
Full CarPlay support for Nextcloud Talk, similar to what WhatsApp and Telegram already offer:

1. Nextcloud Talk appears in the CarPlay app list under iPhone Settings → General → CarPlay
2. A dedicated Talk screen is available in CarPlay showing the conversation list
3. Incoming chat messages are displayed in CarPlay and can be listened to via text-to-speech
4. Users can reply to messages using Siri dictation without touching the phone
5. Voice calls can be initiated and received directly from the CarPlay interface
6. Incoming call notifications appear on the CarPlay screen with accept/decline buttons

**Describe alternatives you've considered**
There is currently no viable alternative. Siri can read notification banners but cannot integrate with the Talk app interface, cannot show the conversation list, and does not allow replying. The phone must be unlocked and handled manually, which defeats the purpose of CarPlay.

**Additional context**
- Apple provides official CarPlay messaging and calling APIs (`CPMessageListItem`, `CPGridTemplate`, `CPCallObserver`) available since iOS 14, which are specifically designed for this use case
- WhatsApp and Telegram are examples of apps that have successfully implemented full CarPlay support using these APIs
- A similar request was made for Android Auto support on talk-android (#3881)
- This feature would significantly improve road safety for Nextcloud Talk users

Start by reviewing the Apple CarPlay APIs named in the issue: CPMessageListItem, CPGridTemplate, and CPCallObserver, along with the existing Talk iOS messaging and calling entry points. Done means Talk appears in CarPlay, supports conversation browsing, message listening and Siri replies, and provides CarPlay call initiation, reception, and controls.

Written by the indexing model from the issue text.