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/05-units-variables.json

@@ -22,7 +22,7 @@
 				{
 					"type": "property_value",
 					"value": { "property": "max-width", "expected": "40rem" },
-					"message": "Set <kbd>max-width: 40rem</kbd>"
+					"message": "Which property caps an element's width? Try a <kbd>rem</kbd> value for readable line length."
 				}
 			]
 		},
@@ -71,7 +71,7 @@
 				{
 					"type": "regex",
 					"value": "width:\\s*calc\\(\\s*100%\\s*-\\s*200px\\s*\\)",
-					"message": "Set <kbd>width: calc(100% - 200px)</kbd>",
+					"message": "Use <kbd>calc()</kbd> to subtract the sidebar's fixed width from the full container width.",
 					"options": { "caseSensitive": false }
 				}
 			]
@@ -93,7 +93,7 @@
 				{
 					"type": "property_value",
 					"value": { "property": "min-height", "expected": "100vh" },
-					"message": "Set <kbd>min-height: 100vh</kbd>"
+					"message": "Which property ensures a minimum height? Use a viewport unit for full-screen coverage."
 				}
 			]
 		}