gallery animation : reveal only once all images are loaded. related to #55

Preload unique image URLs via new Image(). Container stays at opacity:0
until all are ready, then reveals at once. Cached images show instantly
(no fade transition). Fixes broken progressive reveal in scrolling gallery.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/ui/GalleryAnimation.svelte

@@ -7,8 +7,35 @@
    * @prop {number} secondsPerImage — durée par image (défaut: 8s)
    * @prop {'vertical'|'horizontal'} mode — direction du défilement
    */
+  import { onMount } from 'svelte'
+
   let { images = [], secondsPerImage = 8, backgroundColor = null, backgroundImage = null, mode = 'vertical' } = $props()
 
+  let containerEl = $state(null)
+  let allLoaded = $state(false)
+  let needsFade = $state(false)
+
+  // Wait for all unique source images to load before revealing
+  onMount(() => {
+    const uniqueSrcs = new Set(images.map(img => img.src).filter(Boolean))
+    if (uniqueSrcs.size === 0) { allLoaded = true; return }
+
+    let remaining = uniqueSrcs.size
+    const done = () => { if (--remaining <= 0) allLoaded = true }
+
+    for (const src of uniqueSrcs) {
+      const img = new Image()
+      img.src = src
+      if (img.complete) {
+        done()
+      } else {
+        needsFade = true
+        img.onload = done
+        img.onerror = done
+      }
+    }
+  })
+
   const columns = $derived.by(() => {
     const count    = images.length
     const duration = count * secondsPerImage
@@ -40,6 +67,8 @@
 
 <div
   class="gallery-animation gallery-animation--{mode}"
+  class:gallery-animation--ready={allLoaded}
+  class:gallery-animation--fade={needsFade}
   style="--gallery-duration: {columns[0]?.duration ?? 24}s{backgroundColor ? `; --background-color: ${backgroundColor}` : ''}{backgroundImage ? `; --gallery-background-image: url(${backgroundImage})` : ''}"
 >
 <div class="gallery-animation__overlay"></div>
@@ -80,9 +109,17 @@
     overflow: hidden;
     background-color: var(--background-color, transparent);
     background-image: var(--gallery-background-image);
+    opacity: 0;
+  }
+
+  .gallery-animation--ready {
     opacity: .8;
   }
 
+  .gallery-animation--fade.gallery-animation--ready {
+    transition: opacity 0.4s ease;
+  }
+
   .gallery-animation__overlay {
     position: absolute;
     inset: 0;