fix: Replace broken media thumbnails with MediaThumbnail component

Old Cloudinary URLs no longer load (account full). The new MediaThumbnail
component handles broken URLs gracefully with a fallback icon, and uses
<video preload="metadata"> for local video files to show the first frame.

- New component: client/src/components/media-thumbnail.tsx
- Updated: new-reel.tsx, new-post.tsx, media-upload.tsx,
           edit-scheduled-post-dialog.tsx

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

client/src/components/media-thumbnail.tsx

@@ -0,0 +1,57 @@
+import { useState } from 'react';
+import { ImageIcon, VideoIcon } from 'lucide-react';
+
+interface MediaThumbnailProps {
+  src: string;
+  alt: string;
+  type?: 'image' | 'video';
+  className?: string;
+}
+
+function isVideoUrl(url: string): boolean {
+  return /\.(mp4|mov|avi|webm|mkv|m4v)(\?.*)?$/i.test(url);
+}
+
+/**
+ * Renders a media thumbnail that handles:
+ * - Local video files: uses <video preload="metadata"> to show first frame
+ * - Images: standard <img> with lazy loading
+ * - Broken/unavailable URLs: shows a placeholder icon
+ */
+export function MediaThumbnail({ src, alt, type, className = "w-full h-full object-cover" }: MediaThumbnailProps) {
+  const [error, setError] = useState(false);
+  const isVideo = type === 'video' || isVideoUrl(src);
+
+  if (error || !src) {
+    return (
+      <div className={`flex items-center justify-center bg-muted ${className.replace('object-cover', '')}`}>
+        {isVideo
+          ? <VideoIcon className="w-8 h-8 text-muted-foreground" />
+          : <ImageIcon className="w-8 h-8 text-muted-foreground" />
+        }
+      </div>
+    );
+  }
+
+  if (isVideo) {
+    return (
+      <video
+        src={src}
+        className={className}
+        preload="metadata"
+        muted
+        onError={() => setError(true)}
+      />
+    );
+  }
+
+  return (
+    <img
+      src={src}
+      alt={alt}
+      className={className}
+      loading="lazy"
+      onError={() => setError(true)}
+    />
+  );
+}