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/23-html-details-summary.json

@@ -14,7 +14,7 @@
 			"previewHTML": "",
 			"previewBaseCSS": "body { font-family: system-ui; padding: 20px; } details { border: 1px solid #ddd; border-radius: 8px; padding: 15px; } summary { font-weight: 600; cursor: pointer; } summary:hover { color: #1976d2; } details p { margin-top: 15px; color: #666; }",
 			"sandboxCSS": "",
-			"initialCode": "<!-- Create your disclosure widget here -->",
+			"initialCode": "",
 			"solution": "<details>\n  <summary>Click to reveal</summary>\n  <p>This content was hidden!</p>\n</details>",
 			"previewContainer": "preview-area",
 			"validations": [
@@ -67,7 +67,7 @@
 			"previewHTML": "",
 			"previewBaseCSS": "body { font-family: system-ui; min-height: 100vh; background: linear-gradient(135deg, #ffecd2 0%, #fcb69f 100%); display: flex; flex-direction: column; justify-content: center; padding: 40px; margin: 0; box-sizing: border-box; } h1 { font-size: 2.5rem; color: #4a4a4a; text-align: center; margin: 0 0 30px 0; } details { background: white; border-radius: 12px; margin-bottom: 15px; padding: 20px; box-shadow: 0 4px 15px rgba(0,0,0,0.1); } summary { font-size: 1.3rem; font-weight: 600; color: #5c5c5c; cursor: pointer; list-style: none; } summary::before { content: '▸ '; color: #fcb69f; } details[open] summary::before { content: '▾ '; } details p { margin: 15px 0 0 0; color: #666; line-height: 1.6; }",
 			"sandboxCSS": "",
-			"initialCode": "<!-- Create your FAQ section here -->",
+			"initialCode": "",
 			"solution": "<h1>Frequently Asked Questions</h1>\n\n<details>\n  <summary>What is HTML5?</summary>\n  <p>HTML5 is the latest version of HTML with new semantic elements and APIs.</p>\n</details>\n\n<details>\n  <summary>Do I need JavaScript?</summary>\n  <p>Many interactive features work with pure HTML5, no JavaScript required!</p>\n</details>\n\n<details>\n  <summary>Is this accessible?</summary>\n  <p>Yes! Native HTML elements have built-in keyboard and screen reader support.</p>\n</details>",
 			"previewContainer": "preview-area",
 			"validations": [