refactor: add HTML/CSS prefixes to German module titles and improve UI

- Shorten German module titles with consistent HTML/CSS prefixes
- Add .completion-badge styling for the lesson title completion badge
- Make sidebar lessons section fill available height on desktop
- Improve flexbox lesson descriptions with more context

lessons/de/20-html-elements.json

@@ -1,7 +1,7 @@
 {
 	"$schema": "../../schemas/code-crispies-module-schema.json",
 	"id": "html-elements",
-	"title": "HTML-Elemente: Block vs Inline",
+	"title": "HTML Block & Inline",
 	"description": "Verstehe den grundlegenden Unterschied zwischen Container- (Block-) und Inline-Elementen",
 	"mode": "html",
 	"difficulty": "beginner",