Add rich-text / WYSIWYG editor for event descriptions

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

The event description field in Nextcloud Calendar currently only supports plain text.

For meetings and appointments, descriptions often contain structured information such as an agenda, preparation instructions, links, or notes. Without basic formatting, longer descriptions are difficult to structure and read.

It would be helpful to have basic formatting options such as bold, italic, bullet/numbered lists, and links directly in the event editor.

Describe the solution you'd like

I would like the event description field to provide a simple rich-text/WYSIWYG editor with a formatting toolbar.

Basic formatting could include:

Bold and italic
Bullet and numbered lists
Links
Optionally headings

Ideally, this could use existing Nextcloud rich-text/editor components to provide a consistent user experience across Nextcloud.

Compatibility with CalDAV clients should be preserved, so events should still have a plain-text representation where necessary.

Describe alternatives you've considered

Currently, the alternative is to use plain text and manually structure descriptions using characters such as -, *, or line breaks.

Another alternative is to create formatted event descriptions in external calendar clients that support rich-text descriptions, but this does not allow users to create or edit the formatting directly in the Nextcloud Calendar web interface.

Markdown could also be an alternative, but a small WYSIWYG toolbar would be more accessible for users who are not familiar with Markdown syntax.

Additional context

There has previously been discussion about formatted calendar descriptions and HTML alternatives in #3863.

This feature request is specifically about being able to create, edit, and display formatted event descriptions directly in the Nextcloud Calendar web UI.

A possible implementation could retain a plain-text DESCRIPTION for interoperability while additionally supporting a formatted representation where appropriate. However, the exact storage/CalDAV implementation could be decided based on what provides the best compatibility with other calendar clients.

Similar rich-text editing functionality already exists elsewhere in Nextcloud, so reusing existing Nextcloud editor components might also provide a consistent editing experience.

No files or tests are named. Start by reviewing issue #3863 and the existing event editor alongside reusable Nextcloud rich-text components; define the formatting UI, formatted representation, and plain-text CalDAV behavior. Done means users can create, edit, and display formatted descriptions while preserving interoperability.