add slide content improvements and dev server setup

223015b:
- add WTF hex code explanation slide (89=137 decimal, PNG signature)
- add ASCII lead slide with historical context
- remove Hilbert-Studie reference from title

223015c termin 2:
- add OSI layer 5 (session) and layer 6 (presentation) slides
- add URL/domain anatomy slide
- mark HTTP/S section as klausur
- improve status codes formatting with client/server examples
- add CRUD column to HTTP methods table

infrastructure:
- add dev-server.sh for multi-course development
- update generate-index.sh with course-specific colors
- add QR codes for slide URLs

courses/223015b/slides/2026-01-23-termin-3-speichermedien-schnittstellen.md

@@ -58,7 +58,14 @@ Hochschule der Medien Stuttgart
 
 **Wintersemester 2025/26**
 
-[https://git.librete.ch/hdm/223015b](https://git.librete.ch/hdm/223015b)
+[https://librete.ch/hdm/223015b/](https://librete.ch/hdm/223015b/)
+
+---
+
+<!-- _header: '' -->
+<!-- _footer: '' -->
+
+![bg fit](./assets/qr/slides-223015b.png)
 
 ---