Refactor: header sémantique + backdrop limité à expertise/about/blog

- Header : <nav> enveloppée dans <header> (landmark sémantique)
- Backdrop navbar restreint aux slides about et blog (scroll > 100px)
- Portfolio : $effect navbar backdrop supprimé + import navigation retiré

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

src/views/Portfolio.svelte

@@ -1,7 +1,6 @@
 <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'
@@ -18,10 +17,6 @@
   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)
 
@@ -241,10 +236,11 @@
     color: var(--color-primary);
   }
 
-  .portfolio-description {
-    font-size: var(--font-size-subtitle);
-    line-height: 1.5;
+  .portfolio-description :global(p) {
+    font-size: var(--font-size-paragraph-mobile);
+    line-height: 1.4;
     opacity: 0.8;
+    letter-spacing: -2%;
   }
 
   .portfolio-keywords {