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/PopupOnboarding.astro

@@ -0,0 +1,11 @@
+---
+// 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>