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/11-filters.json

@@ -22,7 +22,7 @@
 				{
 					"type": "property_value",
 					"value": { "property": "filter", "expected": "blur(4px)" },
-					"message": "Set <kbd>filter: blur(4px)</kbd>"
+					"message": "Which CSS property applies visual effects like blur? Use the <kbd>blur()</kbd> function with a pixel value."
 				}
 			]
 		},
@@ -48,7 +48,7 @@
 				{
 					"type": "contains",
 					"value": "100%",
-					"message": "Set to <kbd>100%</kbd> for full grayscale"
+					"message": "What percentage value removes all color completely?"
 				}
 			]
 		},
@@ -74,7 +74,7 @@
 				{
 					"type": "contains",
 					"value": "120%",
-					"message": "Set to <kbd>120%</kbd>"
+					"message": "What percentage makes the element slightly brighter than normal? Normal is 100%."
 				}
 			]
 		},
@@ -100,7 +100,7 @@
 				{
 					"type": "contains",
 					"value": "4px 4px 8px",
-					"message": "Set shadow offset and blur"
+					"message": "Set the x-offset, y-offset, and blur radius. The task describes the exact values needed."
 				}
 			]
 		}