fix(frontend): keep empty list item caret aligned

[codekoala]

What changed

Updates the Milkdown editor blank-line CSS so it no longer applies to ProseMirror’s internal trailing break node.

The editor still preserves intentional blank lines, but excludes br.ProseMirror-trailingBreak from the custom block-style <br> rule.

Why

When creating a new Markdown bullet item, ProseMirror inserts an internal trailing break into the empty list item so the caret has a place to render. Windshift’s editor CSS was styling all <br> elements as block-level blank-line placeholders with extra top margin.

That caused the caret for a new empty bullet point to appear offset/misaligned until text was typed. Once text existed, ProseMirror no longer relied on that trailing break, so the alignment looked normal again.

Testing

• fnm exec --using=24.18.0 npm run check
• Verified locally in a deployed Windshift instance by creating a new empty bullet item in a work item description.

Screenshots

Caret offset for empty item in bullet list:

Caret offset for empty item in numbered list:

After this change is applied:

[stefan-ernst]

Great, thank you! Lets get it merged