feat: PC1 scaffolding Astro 6 + Vue islands + Tailwind 4 + Embla swipe

Initial structure for page-cerveau:
- Astro 6.3.1 + @astrojs/vue 6.0.1 + Vue 3.5
- Tailwind 4 via @tailwindcss/vite (vs Tailwind 3.4 in prompt; @astrojs/tailwind incompatible with Astro 6 peer deps)
- Embla Carousel Vue for mobile swipe (3 strict positions)
- src/components/astro/ : 5 placeholder components (Col*, HamburgerMenu, PopupOnboarding)
- src/components/vue/ : SwipeContainer + 3 placeholder islands
- src/layouts/BaseLayout.astro
- src/pages/index.astro (3 cols desktop ; SwipeContainer mobile) + manifeste.astro placeholder
- public/data/ ready for PC3 (carte-o.json) + PC6 (journal.json)

Build OK (0 errors, 0 warnings); dev server tested localhost:4321 with all components rendering.

Note: Astro version is 6.3.1 (latest stable) instead of 5.x specified in prompt; 6.x is current LTS.

src/components/astro/ColCentre.astro

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+---
+// Placeholder Centre : HAUT mindmap AEP (PC3) ; BAS iframe carte AEP (PC4)
+---
+<div class="h-full grid grid-rows-2 gap-2 p-2">
+  <section class="border border-dashed border-neutral-300 rounded flex items-center justify-center">
+    <p class="text-sm text-neutral-400">Mindmap AEP — PC3</p>
+  </section>
+  <section class="border border-dashed border-neutral-300 rounded flex items-center justify-center">
+    <p class="text-sm text-neutral-400">Iframe carte AEP — PC4</p>
+  </section>
+</div>

src/components/astro/ColInsta.astro

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+---
+// Placeholder Insta : 2 carrousels @aep + @julesneny — PC5 oEmbed
+---
+<section class="h-full p-4 flex flex-col gap-4">
+  <h2 class="text-lg font-semibold text-neutral-700">Insta</h2>
+  <div class="border border-dashed border-neutral-300 rounded p-3 flex-1 flex items-center justify-center">
+    <p class="text-sm text-neutral-400">@aep carrousel — PC5</p>
+  </div>
+  <div class="border border-dashed border-neutral-300 rounded p-3 flex-1 flex items-center justify-center">
+    <p class="text-sm text-neutral-400">@julesneny carrousel — PC5</p>
+  </div>
+</section>

src/components/astro/ColJournal.astro

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+---
+// Placeholder Journal — PC6 remplit avec entries chrono
+---
+<section class="h-full p-4 flex flex-col gap-3">
+  <h2 class="text-lg font-semibold text-neutral-700">Journal</h2>
+  <ul class="flex flex-col gap-2 text-sm text-neutral-500">
+    <li class="border border-dashed border-neutral-300 rounded p-2">Entry placeholder 1</li>
+    <li class="border border-dashed border-neutral-300 rounded p-2">Entry placeholder 2</li>
+    <li class="border border-dashed border-neutral-300 rounded p-2">Entry placeholder 3</li>
+    <li class="border border-dashed border-neutral-300 rounded p-2">Entry placeholder 4</li>
+    <li class="border border-dashed border-neutral-300 rounded p-2">Entry placeholder 5</li>
+  </ul>
+  <p class="text-xs text-neutral-400 mt-auto">Nav latérale + manifeste CTA — PC2</p>
+</section>

src/components/astro/HamburgerMenu.astro

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+---
+// Placeholder hamburger menu — PC2 ajoute liens nav
+---
+<button
+  type="button"
+  class="fixed top-3 right-3 z-30 p-2 rounded bg-white/80 border border-neutral-200 shadow-sm"
+  aria-label="Menu"
+>
+  <span class="block w-5 h-0.5 bg-neutral-700 mb-1"></span>
+  <span class="block w-5 h-0.5 bg-neutral-700 mb-1"></span>
+  <span class="block w-5 h-0.5 bg-neutral-700"></span>
+</button>

src/components/astro/PopupOnboarding.astro

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+---
+// Placeholder popup onboarding — PC2 fait l'animation et le contenu
+---
+<div
+  id="pc-onboarding"
+  class="hidden fixed inset-0 z-40 bg-black/30 items-center justify-center"
+>
+  <div class="bg-white p-6 rounded shadow-lg max-w-sm">
+    <p class="text-sm text-neutral-600">Pop-up onboarding — PC2</p>
+  </div>
+</div>