fix(dashboard): override font-weight for bold markdown in dashboard widgets

Add font-weight: 600 override specifically for <strong> tags in dashboard
markdown widgets to restore proper bold text appearance.

The global theme uses fontWeightStrong: 500 (medium), which is too subtle
to distinguish from normal text (400). This scoped override restores the
previous behavior of using font-weight 600 (semibold) for bold markdown
without affecting other components.

Fixes #35485

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: sadpandajoe

superset-frontend/src/dashboard/components/gridComponents/Markdown/Markdown.jsx

@@ -96,6 +96,10 @@ const MarkdownStyles = styled.div`
         font-weight: ${theme.fontWeightNormal};
       }
 
+      strong {
+        font-weight: 600;
+      }
+
       h6 {
         font-size: ${theme.fontSizeSM}px;
       }