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/223015b/slides/2025-12-19-termin-0-intro.md

@@ -42,6 +42,12 @@ code {
 a {
   color: var(--color-highlight);
 }
+section.klausur {
+  background: #e3f2fd !important;
+}
+section.klausur footer {
+  display: none;
+}
 </style>
 
 <!-- _class: invert -->
@@ -69,7 +75,7 @@ Hochschule der Medien Stuttgart
 <!-- _class: lead -->
 
 # Willkommen!
-## Termin 1 – Einführung & Grundlagen
+## Einführung
 
 ---