fix(editor): preserve leading newlines in plain text editor

[omnizs38]

📝 Summary

• Resolves: N/A

This PR fixes an issue where the plain text editor stripped leading newlines and trailing whitespace on empty first lines of plain text documents.

When setting or updating the content of the plain text editor, the raw content is escaped and wrapped inside HTML <pre>...</pre> tags. By standard HTML rules, DOMParser strips a single leading newline character inside <pre> tags. Prepending a newline (\n) to <pre> content ensures that DOMParser strips only our prepended newline, leaving the document's original leading newlines and whitespace intact.

This change also re-enables the previously skipped CommonMark plain text tests 97 and 117, which now pass successfully.

🖼️ Screenshots

🏚️ Before	🏡 After
N/A (No visual changes)	N/A (No visual changes)

🚧 TODO

• None

🏁 Checklist

• Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
• Sign-off message is added to all commits
• Tests (unit, integration and/or end-to-end) passing and the changes are covered with tests
• Documentation (README or documentation) has been updated or is not required

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI tools
• The AI-generated content was reviewed, comprehended and tested by a human

[max-nextcloud]

Thanks a lot @omnizs38

This looks great. A simple fix for a long running problem as the skipped tests indicate.

[max-nextcloud]

static analysis and playwright failures are known and not related.

[max-nextcloud]

/backport to stable34 please

[max-nextcloud]

/backport to stable33 please

[backportbot]

The backport to stable33 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable33
git pull origin stable33

# Create the new backport branch
git checkout -b backport/8787/stable33

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 0580bf9c

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/8787/stable33

Error: Failed to check for changes with origin/stable33: No changes found in backport branch

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

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