fix: add kbd tags to validation messages for clarity

- Wrap HTML element names in <kbd> tags in all validation messages
- Apply consistent formatting to both English and German lessons
- Make hints clearer about which elements/attributes to use
- Fix mobile editor layout overflow issue

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lessons/26-html-data-attributes.json

@@ -31,7 +31,7 @@
 				{
 					"type": "element_count",
 					"value": { "selector": "article h2", "min": 2 },
-					"message": "Add an <h2> inside each article"
+					"message": "Add an <kbd>&lt;h2&gt;</kbd> inside each article"
 				}
 			]
 		},