feat: enhance dashboard and canvas components with credit management features

- Integrated CreditOverview and RecentTransactions components into the dashboard for better credit visibility.
- Updated canvas toolbar to display current credit balance using CreditDisplay.
- Improved AI image and prompt nodes to show credit costs and handle credit availability checks during image generation.
- Added new queries for fetching recent transactions and monthly usage statistics to support dashboard features.
- Refactored existing code to streamline credit-related functionalities across components.

lib/ai-models.ts

@@ -7,6 +7,8 @@ export interface AiModel {
   tier: "budget" | "standard" | "premium";
   description: string;
   estimatedCost: string; // human-readable, e.g. "~€0.04"
+  /** Credits pro Generierung — gleiche Einheit wie Convex reserve/commit (Euro-Cent). */
+  creditCost: number;
   minTier: "free" | "starter" | "pro" | "business"; // minimum subscription tier
 }
 
@@ -17,6 +19,7 @@ export const IMAGE_MODELS: AiModel[] = [
     tier: "standard",
     description: "Fast, high-quality generation",
     estimatedCost: "~€0.04",
+    creditCost: 4,
     minTier: "free",
   },
   // Phase 2 — uncomment when model selector UI is ready:

lib/format-time.ts

@@ -0,0 +1,20 @@
+/**
+ * Formatiert einen Timestamp als relative Zeitangabe.
+ * Beispiele: "Just now", "5m ago", "3h ago", "2d ago", "12. Mär"
+ */
+export function formatRelativeTime(timestamp: number): string {
+  const now = Date.now();
+  const diff = now - timestamp;
+  const minutes = Math.floor(diff / 60000);
+  const hours = Math.floor(diff / 3600000);
+  const days = Math.floor(diff / 86400000);
+
+  if (minutes < 1) return "Just now";
+  if (minutes < 60) return `${minutes}m ago`;
+  if (hours < 24) return `${hours}h ago`;
+  if (days < 7) return `${days}d ago`;
+  return new Date(timestamp).toLocaleDateString("de-DE", {
+    day: "numeric",
+    month: "short",
+  });
+}