feat: integrate credit cost tracking in AI image generation and prompt nodes

- Added credit cost tracking to AI image nodes, displaying the cost in Euro-Cent.
- Updated prompt node to create edges between prompt and AI image nodes during image generation.
- Enhanced Convex action to include credit cost in image generation data handling.
- Introduced utility function for formatting Euro-Cent values for better user display.

lib/utils.ts

@@ -4,3 +4,11 @@ import { twMerge } from "tailwind-merge"
 export function cn(...inputs: ClassValue[]) {
   return twMerge(clsx(inputs))
 }
+
+/** Credits / Preise: Werte sind Euro-Cent (siehe PRD, Manifest). */
+export function formatEurFromCents(cents: number) {
+  return new Intl.NumberFormat("de-DE", {
+    style: "currency",
+    currency: "EUR",
+  }).format(cents / 100)
+}