update slides styling and content refinements

- fix course number 223015c → 223015b in termin 1
- adjust language: "fuck" → "heck" in example
- add black backgrounds for image-only slides
- add bg fit to fullscreen images
- update digital-landscape.png
- add frontmatter to termin 2-4 slides

slides/2025-12-19-termin-0-intro.md

@@ -19,7 +19,6 @@ h2 {
 
 <!-- _class: invert -->
 <!-- _header: '' -->
-<!-- _footer: '' -->
 <!-- _backgroundColor: #000 -->
 
 ![bg fit opacity:0.4](./assets/digital-landscape.png)
@@ -32,7 +31,7 @@ Hochschule der Medien Stuttgart
 
 **Wintersemester 2025/26**
 
-[git.librete.ch/hdm/223015b](https://git.librete.ch/hdm/223015b)
+[https://git.librete.ch/hdm/223015b](https://git.librete.ch/hdm/223015b)
 
 ---