feat: add image preview for large view cards

- Extract YouTube video thumbnails from URLs
- Display thumbnail images as background in large preview cards
- Add gradient overlay for better text contrast
- Fallback to icon placeholder for non-YouTube URLs
- Handle multiple YouTube URL formats (watch, youtu.be, shorts)
- Gracefully handle missing images with icon fallback

src/index.css

@@ -838,16 +838,28 @@ body {
   width: 100%;
   height: 180px;
   background: #1a1a1a;
+  background-size: cover;
+  background-position: center;
+  background-repeat: no-repeat;
   display: flex;
   align-items: center;
   justify-content: center;
   cursor: pointer;
-  transition: background 0.2s ease;
+  transition: all 0.2s ease;
   border-bottom: 1px solid #333;
+  position: relative;
 }
 
 .large-preview-image:hover {
-  background: #222;
+  opacity: 0.9;
+}
+
+.large-preview-image::after {
+  content: '';
+  position: absolute;
+  inset: 0;
+  background: linear-gradient(to bottom, transparent 60%, rgba(0,0,0,0.3) 100%);
+  pointer-events: none;
 }
 
 .preview-placeholder {