fix klausur slides for pdf: add svg pattern and marp directives

- add svg diagonal stripe patterns for pdf background
- use @media print to apply svg only in pdf export
- keep css gradient for web/screen display
- add _header: '' and _footer: '' to all klausur slides

courses/223015c/slides/2026-01-24-termin-3-interaktivitaet-javascript.md

@@ -57,8 +57,11 @@ section.klausur {
     #fff 80px
   ) !important;
 }
-section.klausur footer {
-  display: none;
+@media print {
+  section.klausur {
+    background: url('./assets/klausur-pattern.svg') !important;
+    background-size: cover !important;
+  }
 }
 section.aufgabe {
   background: #fce4ec !important;