feat: add contact section and smooth lesson transitions

- Add contact section to help dialog with librete.ch, GitHub, Gitea, LinkedIn links
- Add HTML/CSS prefixes to English module titles for consistency with German
- Add CSS transitions for smooth lesson switching
- Add transitioning class to prevent content flash during lesson changes

lessons/flexbox.json

@@ -1,7 +1,7 @@
 {
 	"$schema": "../schemas/code-crispies-module-schema.json",
 	"id": "flexbox",
-	"title": "Flexbox",
+	"title": "CSS Flexbox",
 	"description": "Master the flexible box layout model for modern responsive designs",
 	"difficulty": "intermediate",
 	"lessons": [