fix: remove placeholder comments and improve German translations

- Remove placeholder comments from initialCode fields
- Fix typo: "wichtig Wort" -> "wichtigen Wort" in de/20-html-elements.json
- Improve German validation messages for readability
- Use consistent terminology (Eingabefeld instead of Input)

lessons/de/21-html-forms-basic.json

@@ -14,7 +14,7 @@
 			"previewHTML": "",
 			"previewBaseCSS": "body { font-family: system-ui; padding: 20px; } form { max-width: 300px; } label { display: block; margin-bottom: 5px; font-weight: 500; } input { width: 100%; padding: 8px; border: 1px solid #ccc; border-radius: 4px; box-sizing: border-box; }",
 			"sandboxCSS": "",
-			"initialCode": "<!-- Erstelle hier dein Formular -->",
+			"initialCode": "",
 			"solution": "<form>\n  <label for=\"name\">Name:</label>\n  <input type=\"text\" id=\"name\" name=\"name\">\n</form>",
 			"previewContainer": "preview-area",
 			"validations": [