The editor_content CSS class used --page-body-font-weight-bold without
a fallback value. This variable is only injected in the resume preview
context (via use-css-variables.tsx), so in the editor UI the variable
was undefined, causing <strong> tags to render with no visible weight
change.
Adding a 'bold' fallback ensures bold text is correctly displayed in
the editor regardless of whether the CSS variable is set.
Fixes#2730
