style: unify sidebar widget styling with bash comment prefix
All sidebar widgets (author, share, related posts) now use:
- Unified .sidebar-widget container (no box, only top border via <hr>)
- .sidebar-widget-label with # prefix (bash comment style)
- Monospace, dimmed, small text for consistency
Previously, share widget used a different style (full box with borders).
Author and Related Posts widgets now match the hacker aesthetic.
