Make notebook cells follow global font and markdown styling

[JannikRosendahl]

Notebook cells are currently not responding to changes in font-family (zed://settings/buffer_font_family) and font-size (zed://settings/buffer_font_size).

Currently, MarkdownCell and CodeCell create and set a TextStyleRefinement on their Editor, creating copies of font-family and font-size in the process. As a result, these do not get updated when the global font-family or font-size change.
By not setting the refinement manually and letting the editor handle these value instead, these values get updated when the global settings change.

This behaviour is consistent with how the inline repl already behaves and in my opinion is according to the users expectations.

After Review: this PR changes the rendered preview of MarkdownCells to use the themed MarkdownStyle instead of an empty Markdown Style

Before:

origin_main.mov

After:

fix.mov

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
  • not sure if this needs test or how they should look like...
• Performance impact has been considered and is acceptable

Closes #ISSUE

Release Notes:

• Fixed notebook cells not responding to appearance settings changes

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR removes custom text-style refinement configuration from notebook cell editors, likely to let editors inherit styling from defaults/themes and to reduce theme-specific coupling.

Changes:

• Removed ThemeSettings dependency from notebook cells.
• Removed TextStyleRefinement setup (font, size, foreground, transparent background) for Markdown and Code cell editors.
• Cleaned up imports accordingly.

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[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: Jannik Rosendahl.
This is most likely caused by a git client misconfiguration; please make sure to:

1. check if your git client is configured with an email to sign commits git config --list | grep email
2. If not, set it up using git config --global user.email email@example.com
3. Make sure that the git commit email is configured in your GitHub account settings, see https://github.com/settings/emails

[JannikRosendahl]

I have checked markdown rendering in cells after Copilots comments.
I added a second commit with a similar change to the first one, the global markdown preview style is now used. Inline and block code preview now get rendered.

Before:

After:

[MartinYe1234]

Reviewed and the PR is good, small focused change. However, I will be updating the title and description as the the current title and description do not fully capture the scope of the changes.

Thank you for the contribution!