fix(i18n): align German lesson code with English

- Changed solution in 00-welcome.json from "Hallo Welt" to "Hello World"
- Added missing solution fields to 01-box-model.json (8 lessons)
- Added missing solution fields to 05-units-variables.json (4 lessons)
- Added missing solution fields to 06-transitions-animations.json (4 lessons)
- Added missing solution fields to 08-responsive.json (4 lessons)
- Fixed flexbox.json class names from .green/.red/.yellow to .box1/.box2/.box3
- Added missing solution fields to flexbox.json (6 lessons)

German translations now keep all CSS code in English for maintainability,
with only instructional text translated.

lessons/de/00-welcome.json

@@ -10,18 +10,18 @@
 			"id": "get-started",
 			"title": "Los geht's",
 			"description": "<strong>Code Crispies</strong> ist eine kostenlose Open-Source-Plattform zum Erlernen von Webentwicklung durch praktische Übungen. Kein Konto erforderlich!<br><br><strong>Was du lernst:</strong><br>• <strong>HTML</strong> - Semantische Elemente, Formulare, Tabellen, SVG (<em>HTML Block & Inline</em>, <em>HTML Formulare</em>, <em>HTML Tabellen</em>)<br>• <strong>CSS</strong> - Selektoren, Box-Model, Flexbox, Animationen (<em>CSS Selektoren</em>, <em>CSS Box-Model</em>, <em>CSS Flexbox</em>)<br>• <strong>Responsive Design</strong> - Media Queries und Mobile-First Layouts<br><br><strong>So funktioniert's:</strong><br>1. Lies die Aufgabe im linken Bereich<br>2. Schreibe Code im Editor<br>3. Sieh Live-Ergebnisse in der Vorschau<br>4. Bekomme sofortiges Feedback mit Hinweisen<br><br><strong>Tastenkürzel:</strong> <kbd>Strg+Z</kbd> rückgängig, <kbd>Strg+Umschalt+Z</kbd> wiederholen<br><br><strong>Weitere Ressourcen:</strong><br>• <a href=\"https://nextlevelshit.github.io/html-over-js/\" target=\"_blank\">HTML over JS</a> - Native HTML vs JavaScript-Lösungen<br>• <a href=\"https://nextlevelshit.github.io/web-engineering-mandala/\" target=\"_blank\">Web Engineering Mandala</a> - JavaScript Technologie-Roadmap",
-			"task": "Hallo Welt",
+			"task": "Hello World",
 			"previewHTML": "",
 			"previewBaseCSS": "body { font-family: system-ui, sans-serif; padding: 20px; text-align: center; } h1 { color: #6366f1; }",
 			"sandboxCSS": "",
 			"initialCode": "",
-			"solution": "<h1>Hallo Welt</h1>",
+			"solution": "<h1>Hello World</h1>",
 			"previewContainer": "preview-area",
 			"validations": [
 				{
 					"type": "contains",
-					"value": "Hallo",
-					"message": "Schreibe 'Hallo Welt'"
+					"value": "Hello",
+					"message": "Schreibe 'Hello World'"
 				}
 			]
 		},
@@ -53,7 +53,7 @@
 			"previewHTML": "",
 			"previewBaseCSS": "",
 			"sandboxCSS": "",
-			"initialCode": "<style>\n  body {\n    font-family: system-ui, sans-serif;\n    padding: 20px;\n  }\n</style>\n\n<h1>Hallo Welt</h1>\n<p>Los geht's!</p>",
+			"initialCode": "<style>\n  body {\n    font-family: system-ui, sans-serif;\n    padding: 20px;\n  }\n</style>\n\n<h1>Hello World</h1>\n<p>Start coding!</p>",
 			"solution": "",
 			"previewContainer": "preview-area",
 			"validations": []