feat: add concept field to lesson schema

Add 'concept' object to lesson schema with:
- explanation: required string for 2-4 sentence concept explanation
- diagram: optional string for SVG/ASCII art diagrams
- containerVsItem: optional string for Flexbox-specific distinctions

schemas/code-crispies-module-schema.json

@@ -99,6 +99,26 @@
 						"type": "string",
 						"description": "ID of the container element for the preview"
 					},
+					"concept": {
+						"type": "object",
+						"description": "Conceptual explanation of WHY the CSS/HTML works, not just syntax",
+						"properties": {
+							"explanation": {
+								"type": "string",
+								"description": "Beginner-friendly explanation (2-4 sentences) of the concept behind the lesson"
+							},
+							"diagram": {
+								"type": "string",
+								"description": "Optional SVG markup or ASCII art diagram to visualize the concept"
+							},
+							"containerVsItem": {
+								"type": "string",
+								"description": "Optional explanation for Flexbox/Grid lessons to clarify container vs item distinction"
+							}
+						},
+						"required": ["explanation"],
+						"additionalProperties": false
+					},
 					"validations": {
 						"type": "array",
 						"description": "Rules to validate user input",