feat(showcase): improve remote image loading with skeleton animation and fade-in effects

- Add preloadMedia() for async image/video loading before display
- Implement renderLoadingSkeleton() with fa-circle-notch fa-spin animation
- Add fadeIn transition (opacity 0→1) for main media elements
- Remove shimmer gradient animation from thumbnails for cleaner look
- Use solid background color placeholder with subtle fade-in for thumbnails
- Fixes progressive rendering of remote images from top to bottom
- Prevents black flash during loading with proper loading states

static/css/components/model-modal/thumbnail-rail.css

@@ -36,7 +36,19 @@
   position: relative;
   border: 2px solid transparent;
   transition: border-color 0.2s ease, transform 0.2s ease;
-  background: var(--bg-color);
+  background: var(--lora-surface);
+}
+
+.thumbnail-rail__item img {
+  width: 100%;
+  height: 100%;
+  object-fit: cover;
+  opacity: 0;
+  transition: opacity 0.3s ease;
+}
+
+.thumbnail-rail__item img.loaded {
+  opacity: 1;
 }
 
 .thumbnail-rail__item:hover {
@@ -49,12 +61,6 @@
   box-shadow: 0 0 0 2px oklch(var(--lora-accent-l) var(--lora-accent-c) var(--lora-accent-h) / 0.3);
 }
 
-.thumbnail-rail__item img {
-  width: 100%;
-  height: 100%;
-  object-fit: cover;
-}
-
 /* NSFW blur for thumbnails - BEM naming to avoid conflicts with global .nsfw-blur */
 .thumbnail-rail__item--nsfw-blurred img {
   filter: blur(8px);