feat: implement #4 — replace answer-revealing validation messages with pedagogical hints

Rewrite ~120 validation error messages across 17 English lesson modules
and their localized variants (ar, de, es, pl, uk) to use concept questions,
property hints, and directional nudges instead of revealing the exact
CSS property-value answers.

Priority modules (flexbox, box-model, colors, positioning) fully rewritten.
All remaining CSS modules updated. Only message strings changed — no
validation logic modifications.

lessons/12-positioning.json

@@ -22,7 +22,7 @@
 				{
 					"type": "property_value",
 					"value": { "property": "position", "expected": "relative" },
-					"message": "Set <kbd>position: relative</kbd>"
+					"message": "Which position value keeps an element in normal flow but allows offset adjustments?"
 				}
 			]
 		},
@@ -43,7 +43,7 @@
 				{
 					"type": "property_value",
 					"value": { "property": "top", "expected": "-8px" },
-					"message": "Set <kbd>top: -8px</kbd>"
+					"message": "Which offset property moves an element upward from its current position?"
 				}
 			]
 		},
@@ -64,7 +64,7 @@
 				{
 					"type": "property_value",
 					"value": { "property": "position", "expected": "absolute" },
-					"message": "Set <kbd>position: absolute</kbd>"
+					"message": "Which position value removes an element from normal flow for precise placement?"
 				}
 			]
 		},
@@ -85,12 +85,12 @@
 				{
 					"type": "property_value",
 					"value": { "property": "top", "expected": "8px" },
-					"message": "Set <kbd>top: 8px</kbd>"
+					"message": "Which offset property controls the distance from the top of the positioned ancestor?"
 				},
 				{
 					"type": "property_value",
 					"value": { "property": "right", "expected": "8px" },
-					"message": "Set <kbd>right: 8px</kbd>"
+					"message": "Which offset property controls the distance from the right edge?"
 				}
 			]
 		}