feat: use album cover image as album header background

- Fetch cover_image in album query
- Set album-header background-image with cover image
- Add dark overlay (50% opacity) over cover for text readability
- Increased padding for better header spacing with cover image
- Improved visual design using cover image as backdrop
- Fallback to overlay-only design if no cover image exists
- Enhanced header layout with proper z-index for content layering

src/pages/gallery/gallery.php

@@ -171,7 +171,7 @@ $conn->close();
         background: #667eea;
         color: white;
         border: none;
-        border-radius: 6px;
+        border-radius: 30px;
         font-size: 0.85rem;
         font-weight: 600;
         cursor: pointer;