unify slide styles across all termine

223015b:
- add klausur styles to termin-3, 4, 5
- consistent styles across all 6 termine

223015c:
- h1: darker raspberry (#a02060)
- h2: dark gray almost black (#1f2937)
- section.invert h1: white
- consistent styles across all 3 termine

also:
- add squoosh link and QR code
- update video title to "RIIIIIIIESIG"
- replace i-frame diagram with canon image

courses/223015c/slides/2025-12-20-termin-1-geschichte-grundlagen-html.md

@@ -20,8 +20,14 @@ section.invert {
 section {
   font-size: 1.7rem;
 }
+h1 {
+  color: #a02060;  /* darker raspberry */
+}
+section.invert h1 {
+  color: #fff;
+}
 h2 {
-  color: var(--color-highlight);
+  color: #1f2937;  /* dark gray, almost black */
 }
 pre {
   background: #0f0f23;