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/24-html-progress-meter.json

@@ -14,7 +14,7 @@
 			"previewHTML": "",
 			"previewBaseCSS": "body { font-family: system-ui; padding: 20px; } label { display: block; margin-bottom: 8px; font-weight: 500; } progress { width: 100%; height: 20px; border-radius: 10px; } progress::-webkit-progress-bar { background: #e0e0e0; border-radius: 10px; } progress::-webkit-progress-value { background: linear-gradient(90deg, #4caf50, #8bc34a); border-radius: 10px; } progress::-moz-progress-bar { background: linear-gradient(90deg, #4caf50, #8bc34a); border-radius: 10px; }",
 			"sandboxCSS": "",
-			"initialCode": "<!-- Create a progress bar here -->",
+			"initialCode": "",
 			"solution": "<label for=\"download\">Download:</label>\n<progress id=\"download\" value=\"70\" max=\"100\">70%</progress>",
 			"previewContainer": "preview-area",
 			"validations": [
@@ -48,7 +48,7 @@
 			"previewHTML": "",
 			"previewBaseCSS": "body { font-family: system-ui; padding: 20px; } p { margin-bottom: 10px; color: #666; } progress { width: 100%; height: 8px; border-radius: 4px; } progress::-webkit-progress-bar { background: #e0e0e0; border-radius: 4px; }",
 			"sandboxCSS": "",
-			"initialCode": "<!-- Create an indeterminate loading indicator -->",
+			"initialCode": "",
 			"solution": "<p>Loading...</p>\n<progress></progress>",
 			"previewContainer": "preview-area",
 			"validations": [
@@ -72,7 +72,7 @@
 			"previewHTML": "",
 			"previewBaseCSS": "body { font-family: system-ui; padding: 20px; } label { display: block; margin-bottom: 8px; font-weight: 500; } meter { width: 100%; height: 25px; }",
 			"sandboxCSS": "",
-			"initialCode": "<!-- Create a battery meter here -->",
+			"initialCode": "",
 			"solution": "<label for=\"battery\">Battery:</label>\n<meter id=\"battery\" value=\"0.8\" min=\"0\" max=\"1\" low=\"0.2\" high=\"0.8\" optimum=\"1\">80%</meter>",
 			"previewContainer": "preview-area",
 			"validations": [