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/23-html-details-summary.json

@@ -1,7 +1,7 @@
 {
 	"$schema": "../../schemas/code-crispies-module-schema.json",
 	"id": "html-details-summary",
-	"title": "Details & Summary: Aufklapp-Elemente",
+	"title": "HTML Details & Summary",
 	"description": "Erstelle aufklappbare Inhaltsbereiche ohne JavaScript",
 	"mode": "html",
 	"difficulty": "beginner",