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/09-gradients.json

@@ -22,7 +22,7 @@
 				{
 					"type": "contains",
 					"value": "linear-gradient",
-					"message": "Use <kbd>linear-gradient()</kbd>"
+					"message": "Which CSS function creates a smooth transition between colors along a straight line?"
 				},
 				{
 					"type": "contains",
@@ -53,7 +53,7 @@
 				{
 					"type": "contains",
 					"value": "to right",
-					"message": "Add <kbd>to right</kbd> to set the direction"
+					"message": "Which direction keyword makes a gradient flow horizontally from the left side?"
 				}
 			]
 		},
@@ -74,7 +74,7 @@
 				{
 					"type": "contains",
 					"value": "radial-gradient",
-					"message": "Use <kbd>radial-gradient()</kbd>"
+					"message": "Which CSS function creates a gradient that radiates outward from a center point?"
 				},
 				{
 					"type": "contains",