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/pages/index.astro

@@ -0,0 +1,29 @@
+---
+import BaseLayout from '../layouts/BaseLayout.astro';
+import ColJournal from '../components/astro/ColJournal.astro';
+import ColCentre from '../components/astro/ColCentre.astro';
+import ColInsta from '../components/astro/ColInsta.astro';
+import SwipeContainer from '../components/vue/SwipeContainer.vue';
+import HamburgerMenu from '../components/astro/HamburgerMenu.astro';
+import PopupOnboarding from '../components/astro/PopupOnboarding.astro';
+---
+<BaseLayout title="trans-former.fr">
+  <HamburgerMenu />
+  <PopupOnboarding />
+
+  <!-- Desktop : grid 3 colonnes -->
+  <div class="hidden md:grid md:grid-cols-[320px_1fr_320px] h-screen overflow-hidden">
+    <aside class="border-r border-neutral-200 overflow-y-auto"><ColJournal /></aside>
+    <main class="overflow-hidden"><ColCentre /></main>
+    <aside class="border-l border-neutral-200 overflow-y-auto"><ColInsta /></aside>
+  </div>
+
+  <!-- Mobile : SwipeContainer Vue island -->
+  <div class="md:hidden h-screen overflow-hidden">
+    <SwipeContainer client:load>
+      <ColJournal slot="left" />
+      <ColCentre slot="center" />
+      <ColInsta slot="right" />
+    </SwipeContainer>
+  </div>
+</BaseLayout>

src/pages/manifeste.astro

@@ -0,0 +1,9 @@
+---
+import BaseLayout from '../layouts/BaseLayout.astro';
+---
+<BaseLayout title="Manifeste — trans-former.fr">
+  <main class="max-w-2xl mx-auto p-6">
+    <h1 class="text-2xl font-semibold mb-4">Manifeste</h1>
+    <p class="text-sm text-neutral-500">Placeholder — PC2 importe le contenu manifeste depuis astro-site existant.</p>
+  </main>
+</BaseLayout>