Add 'concept' objects to all 4 lessons in 02-selectors.json:
- Element selectors: Explain DOM traversal and specificity (0,0,0,1)
- Class selectors: Explain attribute matching and medium specificity (0,0,1,0)
- ID selectors: Explain uniqueness and high specificity (0,1,0,0)
- Combined selectors: Explain AND logic and specificity addition
Each concept includes:
- Beginner-friendly explanation (2-4 sentences)
- ASCII diagram showing how selectors match
- Specificity comparison and cascade behavior
Subtask 3.5 - Advanced selectors conceptual explanations
- 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)
