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/state/navigation.svelte.js

@@ -1,12 +1,15 @@
 let isMenuOpen = $state(false)
 let isLoading = $state(false)
+let isScrolled = $state(false)
 
 export const navigation = {
   get isMenuOpen() { return isMenuOpen },
   get isLoading() { return isLoading },
+  get isScrolled() { return isScrolled },
 
   toggleMenu: () => isMenuOpen = !isMenuOpen,
   openMenu: () => isMenuOpen = true,
   closeMenu: () => isMenuOpen = false,
-  setLoading: (value) => isLoading = value
+  setLoading: (value) => isLoading = value,
+  setScrolled: (value) => isScrolled = value,
 }