perf(showcase): prefetch adjacent examples and shrink gallery thumbnails

- Warm the HTTP cache for examples adjacent to the active one after
  expand and on every navigation, so prev/next feels instant (images
  only, deduped, low fetch priority)
- Add GALLERY_THUMBNAIL optimization mode (width=160) for the 72px
  gallery strip instead of reusing the 450px card thumbnails
- Hint priorities: fetchpriority=high on the main media, low on
  strip thumbnails

static/js/components/shared/showcase/MediaRenderers.js

@@ -76,6 +76,7 @@ export function generateImageWrapper(media, shouldBlur, nsfwText, metadataPanel,
                 alt="Preview" 
                 width="${media.width}"
                 height="${media.height}"
+                fetchpriority="high"
                 class="lazy ${shouldBlur ? 'blurred' : ''}"> 
             ${shouldBlur ? `
                 <div class="nsfw-overlay">