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/showcase.css

@@ -77,7 +77,12 @@
   max-height: 70vh;
   object-fit: contain;
   border-radius: var(--border-radius-sm);
-  transition: filter 0.2s ease;
+  transition: filter 0.2s ease, opacity 0.3s ease;
+  opacity: 0;
+}
+
+.showcase__media.loaded {
+  opacity: 1;
 }
 
 .showcase__media.blurred {
@@ -384,6 +389,51 @@
   font-size: 2rem;
 }
 
+/* Skeleton loading state */
+.showcase__skeleton {
+  width: 100%;
+  height: 100%;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  background: var(--lora-surface);
+}
+
+.skeleton-animation {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  gap: var(--space-3);
+  color: var(--text-color);
+  opacity: 0.6;
+}
+
+.skeleton-spinner {
+  font-size: 2.5rem;
+  color: var(--lora-accent);
+}
+
+/* Error state */
+.showcase__error {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  height: 100%;
+  color: var(--lora-error);
+  gap: var(--space-2);
+}
+
+.showcase__error i {
+  font-size: 2rem;
+}
+
+.showcase__error p {
+  margin: 0;
+  font-size: 0.9em;
+}
+
 /* Empty state */
 .showcase__empty {
   display: flex;

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);