feat: implement node deletion handling and geometry synchronization in canvas

- Added functionality to block node deletion based on synchronization status with Convex, providing user feedback through toast notifications.
- Introduced helper functions to determine reasons for blocking deletions, enhancing user experience during canvas interactions.
- Updated asset node styling to improve visual consistency and adjusted minimum dimensions for asset nodes to ensure better layout management.

components/canvas/nodes/asset-node.tsx

@@ -199,7 +199,7 @@ export default function AssetNode({ id, data, selected, width, height }: NodePro
               <img
                 src={previewUrl}
                 alt={data.title ?? "FreePik-Vorschau"}
-                className={`h-full w-full object-cover object-right transition-opacity ${
+                className={`h-full w-full object-contain transition-opacity ${
                   isPreviewLoading ? "opacity-0" : "opacity-100"
                 }`}
                 draggable={false}