Added 'concept' objects to 4 lessons in basic selectors module:
- Type + ID: Explains specificity boost from combining selectors
- Selector Lists: Explains OR logic and independent matching
- Universal Selector: Explains wildcard matching and descendant context
- Specificity: Explains cascade and point system for selector precedence
All concepts include beginner-friendly explanations (2-4 sentences) and ASCII diagrams showing how selectors match elements and resolve conflicts.
- Add contact section to help dialog with librete.ch, GitHub, Gitea, LinkedIn links
- Add HTML/CSS prefixes to English module titles for consistency with German
- Add CSS transitions for smooth lesson switching
- Add transitioning class to prevent content flash during lesson changes
- Replace single quotes with <kbd> tags in validation messages
- German and English lessons updated for consistent formatting
- Move task instruction before description in UI (index.html)
