Add shader gradient background with halo effect

- Installed @shadergradient/react, three, and @types/three
- Created ShaderGradientBackground component with custom gradient settings
- Integrated background into root layout with blur effect (120px) and reduced opacity (0.5)
- Adjusted body background to work harmoniously with shader gradient
- Used sphere type with pink/orange gradient colors (#b01040, #b04a17, #e167c5)
- Configured animation, grain, and lighting for atmospheric effect

DIRECTUS_CHECKLIST.md

@@ -167,7 +167,7 @@ consent.hide          | de | Ausblenden
 
 ## Collection: content_pages
 
-Diese sind für **längere Inhalte**. Nutze den Rich-Text-Editor in Directus oder Markdown.
+Diese sind für **längere Inhalte**. Nutze den Ric-Text-Editor in Directus oder Markdown.
 
 ### Home – Hero (langere Beschreibung)
 - **slug**: home-hero