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}

components/canvas/nodes/base-node-wrapper.tsx

@@ -16,7 +16,7 @@ const RESIZE_CONFIGS: Record<string, ResizeConfig> = {
   frame: { minWidth: 200, minHeight: 150 },
   group: { minWidth: 150, minHeight: 100 },
   image: { minWidth: 140, minHeight: 120, keepAspectRatio: true },
-  asset: { minWidth: 140, minHeight: 208, keepAspectRatio: false },
+  asset: { minWidth: 200, minHeight: 240, keepAspectRatio: false },
   video: { minWidth: 200, minHeight: 120, keepAspectRatio: true },
   // Chrome 88 + min. Viewport 120 → äußere Mindesthöhe 208 (siehe canvas onNodesChange)
   "ai-image": { minWidth: 200, minHeight: 208, keepAspectRatio: false },