Feat: navbar frosted glass au scroll

- navigation.svelte.js : ajout isScrolled + setScrolled()
- Header : scroll listener (capture) sur .page-scrollable > 100px,
  reset au changement de slide, classe navbar--scrolled conditionnelle,
  transition 0.4s sur background-color et backdrop-filter
- Expertise : $effect notifie quand currentItem > 0
- Portfolio : $effect notifie quand currentIndex > 0

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

src/views/Portfolio.svelte

@@ -1,6 +1,7 @@
 <script>
   import { onMount } from 'svelte'
   import { slides } from '@state/slides.svelte'
+  import { navigation } from '@state/navigation.svelte'
   import { createScrollNav } from '@composables/useScrollNav.svelte.js'
   import GalleryAnimation from '@components/ui/GalleryAnimation.svelte'
   import ResponsivePicture from '@components/ui/ResponsivePicture.svelte'
@@ -17,6 +18,10 @@
   const backgroundImage   = $derived(data?.backgroundImage ?? null)
   const currentProject = $derived(projects[currentIndex] ?? null)
 
+  $effect(() => {
+    if (isActive) navigation.setScrolled(currentIndex > 0)
+  })
+
   // Capture du hash synchrone avant que tout effect puisse le modifier
   const initialHash = window.location.hash.slice(1)