rename background images, split summary slide, remove assets.md

- rename background images to background-termin-{1,2,3}.png
- split termin 1 summary slide to fix overflow
- remove ASSETS.md (wrong project file)

slides/2026-01-24-termin-3-interaktivitaet-javascript.md

@@ -21,7 +21,7 @@ h2 {
 <!-- _header: '' -->
 <!-- _footer: '' -->
 
-![bg fit opacity:0.4](./assets/javascript-code.png)
+![bg fit opacity:0.4](./assets/background-termin-3.png)
 
 # Grundlagen IT- und Internettechnik