fix: update sticky headers and explore border for flex layout
- Change notifications header sticky position from top: 48px to top: 0
- Change explore search container sticky position from top: 48px to top: 0
- Constrain explore page border between search and tabs to feed max-width
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>