fix(dashboard): restore blue table headers broken by get_stylesheet change
get_stylesheet was changed to return a plain URL string for Panel 1.9.x compatibility, but Bokeh DataTable.stylesheets serialises strings as inline CSS — the URL path is never fetched so the custom-bokeh.css styles are lost.
Add get_bokeh_stylesheet() which wraps the URL in ImportedStyleSheet, and use it for all Bokeh DataTable stylesheets. Panel Tabulator widgets keep using the plain string form.
Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com
Closes #379 (closed)