fix: derive original cover image key in EditPost component from fetched data or URL

src/pages/EditPost.tsx

@@ -3,6 +3,7 @@ import { useNavigate, useParams } from 'react-router-dom';
 import Layout from '../components/Layout';
 import BlogEditor, { type BlogEditorRef } from '../blog/BlogEditor';
 import { getBlogPost, updateBlogPost, uploadImage } from '../blog/api';
+import { extractFileKeyFromUrl } from '../blog/s3Config';
 import { Toast } from '../components/Toast';
 import { useToast } from '../hooks/useToast';
 import '../App.css';
@@ -348,7 +349,11 @@ function EditPost() {
       setTitle(fetched.title);
       setInitialContent(fetched.content);
       setCoverImage(fetched.coverImageUrl ?? null);
-      setOriginalCoverImageKey(fetched.coverImageKey ?? '');
+
+      const derivedCoverKey =
+        fetched.coverImageKey ??
+        (fetched.coverImageUrl ? extractFileKeyFromUrl(fetched.coverImageUrl) ?? '' : '');
+      setOriginalCoverImageKey(derivedCoverKey);
       setNewCoverImageKey(null);
     } catch (err) {
       const message = err instanceof Error ? err.message : 'Failed to load post.';