feat(blocks): add icon support to CardGridBlock

Cards can now display a Lucide icon as alternative to an image,
with configurable position (top/left). Fields show conditionally
based on mediaType selection.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/blocks/CardGridBlock.ts

@@ -20,11 +20,47 @@ export const CardGridBlock: Block = {
       minRows: 1,
       maxRows: 6,
       fields: [
+        {
+          name: 'mediaType',
+          type: 'select',
+          defaultValue: 'none',
+          label: 'Visuelles Element',
+          options: [
+            { label: 'Keins', value: 'none' },
+            { label: 'Bild', value: 'image' },
+            { label: 'Icon', value: 'icon' },
+          ],
+        },
         {
           name: 'image',
           type: 'upload',
           relationTo: 'media',
           label: 'Bild',
+          admin: {
+            condition: (_, siblingData) => siblingData?.mediaType === 'image',
+          },
+        },
+        {
+          name: 'icon',
+          type: 'text',
+          label: 'Icon-Name',
+          admin: {
+            description: 'Lucide Icon-Name (z.B. "heart", "star", "shield-check", "camera")',
+            condition: (_, siblingData) => siblingData?.mediaType === 'icon',
+          },
+        },
+        {
+          name: 'iconPosition',
+          type: 'select',
+          defaultValue: 'top',
+          label: 'Icon-Position',
+          options: [
+            { label: 'Oben', value: 'top' },
+            { label: 'Links', value: 'left' },
+          ],
+          admin: {
+            condition: (_, siblingData) => siblingData?.mediaType === 'icon',
+          },
         },
         {
           name: 'title',