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

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