Add user mentions to card descriptions

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Is your feature request related to a problem? Please describe.
Currently, users can be mentioned in comments on Deck cards, but there is no equivalent mention functionality for the card description.

This would be especially useful for longer card descriptions containing tasks, notes, or responsibilities, for example:

• @alice please review this section
• @bob please provide the required information

The Deck description editor already provides the Person option through the / command / Smart Picker. However, inserting a person into the description does not behave like an actual mention and does not notify the mentioned user.

Describe the solution you'd like
It would be great if Deck supported real user mentions inside card descriptions, similar to mentions in comments.

Ideally:

1. Typing @ in a card description should allow searching for and selecting Nextcloud users.
2. Alternatively, selecting / → Person should insert the selected user as a mention.
3. The mentioned user should receive a Nextcloud notification such as:
   Alice mentioned you in the description of card "Example card".
4. Clicking the notification should open the corresponding Deck card.
5. Mentions should be visually distinguishable from normal text and link to the mentioned user where appropriate.

Current behavior

Mentions and corresponding notifications are currently supported in Deck comments.

In card descriptions, the editor exposes person-related functionality, but Deck does not appear to treat inserted users as mentions that trigger notifications.

Expected behavior

Users mentioned in a card description should be recognized by Deck and receive a notification, similar to mentions in card comments.

Start by tracing how mentions and notifications work in Deck card comments, then inspect the card description editor's Person option and Smart Picker behavior. The work is done when descriptions recognize selected users as mentions, notify them, provide a link back to the card, and distinguish mentions visually as requested.