vertical lines : centraliser en fixed dans App.svelte

- Retirer les lignes des views (Home, Expertise, About, Portfolio)
- Centraliser dans App.svelte en position:fixed, visibilité par slide
  via transition de hauteur 0→100vh selon slides.active.template
- Positions calculées depuis la golden-grid (1/6, 1/3, 1/2, 2/3, 5/6)
- Nettoyer layout.css (garder seulement les classes pour Menu.svelte)

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

src/App.svelte

@@ -36,6 +36,15 @@
   const wrapperWidth = $derived(`${slides.all.length * 100}vw`)
   const wrapperTransform = $derived(`translateX(-${slides.activeIndex * 100}vw)`)
 
+  const linesBySlide = {
+    home:      [6, 11, 16],
+    expertise: [6, 8, 11, 14, 16],
+    about:     [6, 8, 11, 14, 16],
+    portfolio: [11, 16],
+  }
+  const ALL_COLS = [6, 8, 11, 14, 16]
+  const activeLines = $derived(new Set(linesBySlide[slides.active?.template] ?? []))
+
   let isReady    = $state(false)
   let isResizing = $state(false)
   let resizeTimer = null
@@ -127,6 +136,12 @@
 <Header />
 <div class="bg-fixed" style="background-image: url('/assets/img/scrollable-page-background.png')"></div>
 
+<div class="vertical-lines" aria-hidden="true">
+  {#each ALL_COLS as col}
+    <div class="vl vl-col{col}" class:visible={activeLines.has(col)}></div>
+  {/each}
+</div>
+
 <main class="main">
   <div
     class="slides-wrapper"
@@ -204,4 +219,28 @@
     background-position: center;
     background-repeat: no-repeat;
   }
+
+  .vertical-lines {
+    position: fixed;
+    inset: 0;
+    z-index: var(--z-base);
+    pointer-events: none;
+  }
+
+  .vl {
+    position: absolute;
+    top: 0;
+    height: 0;
+    border-left: 0.1px solid rgba(238, 238, 238, 0.2);
+    overflow: hidden;
+    transition: height 600ms cubic-bezier(0.77, 0, 0.175, 1);
+  }
+
+  .vl.visible { height: 100vh; }
+
+  .vl-col6  { left: calc(10.66 / 63.96 * 100%); }
+  .vl-col8  { left: calc(21.32 / 63.96 * 100%); }
+  .vl-col11 { left: calc(31.98 / 63.96 * 100%); }
+  .vl-col14 { left: calc(42.64 / 63.96 * 100%); }
+  .vl-col16 { left: calc(53.30 / 63.96 * 100%); }
 </style>