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/223015b/slides/assets/klausur-pattern.svg

@@ -0,0 +1,9 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="80" height="80">
+  <defs>
+    <pattern id="diagonalStripes" patternUnits="userSpaceOnUse" width="80" height="80" patternTransform="rotate(135)">
+      <rect width="40" height="80" fill="#e3f2fd"/>
+      <rect x="40" width="40" height="80" fill="#ffffff"/>
+    </pattern>
+  </defs>
+  <rect width="100%" height="100%" fill="url(#diagonalStripes)"/>
+</svg>