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/13-pseudo-elements.json

@@ -48,7 +48,7 @@
 				{
 					"type": "property_value",
 					"value": { "property": "color", "expected": "coral" },
-					"message": "Set <kbd>color: coral</kbd>"
+					"message": "Which CSS property changes the text color of the bullet? Try a warm, pinkish-orange named color."
 				}
 			]
 		},
@@ -95,17 +95,17 @@
 				{
 					"type": "property_value",
 					"value": { "property": "width", "expected": "40px" },
-					"message": "Set <kbd>width: 40px</kbd>"
+					"message": "How wide should the decorative line be? Check the task for the pixel value."
 				},
 				{
 					"type": "property_value",
 					"value": { "property": "height", "expected": "3px" },
-					"message": "Set <kbd>height: 3px</kbd>"
+					"message": "Which CSS property controls the thickness of the line? A thin line looks best here."
 				},
 				{
 					"type": "property_value",
 					"value": { "property": "background", "expected": "steelblue" },
-					"message": "Set <kbd>background: steelblue</kbd>"
+					"message": "Which CSS property fills the line with color? Use a steel-toned blue named color."
 				}
 			]
 		}