global : preload bg-fixed background image before showing. related to #55

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

src/App.svelte

@@ -52,9 +52,18 @@
 
   let isReady    = $state(false)
   let isResizing = $state(false)
+  let bgReady    = $state(false)
   let resizeTimer = null
   let mediaCheckCleanup = null
 
+  // Preload bg-fixed image, show only once fully loaded
+  const BG_URL = '/assets/img/scrollable-page-background.png'
+  {
+    const img = new Image()
+    img.onload = () => { bgReady = true }
+    img.src = BG_URL
+  }
+
   // Active la transition seulement après le premier paint à la bonne position.
   // Double rAF : le premier laisse passer un paint avec le bon translateX,
   // le second active is-animated — évite l'animation parasite au chargement.
@@ -219,7 +228,9 @@
 
 <Cursor />
 <Header />
-<div class="bg-fixed" style="background-image: url('/assets/img/scrollable-page-background.png')"></div>
+{#if bgReady}
+  <div class="bg-fixed" style="background-image: url('{BG_URL}')"></div>
+{/if}
 
 <div class="vertical-lines" aria-hidden="true">
   {#each ALL_COLS as col}