feat: scaffold Astro project with layout, nav, hero and concept sections

- Add global ocean-themed styles (Inter + Sora, glassmorphism utilities).
- Wire up base layout, sticky nav and footer respecting base path.
- Build hero with stylised rock SVG + sonar ambient rings and concept pipeline schematic.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/Nav.astro

@@ -0,0 +1,45 @@
+---
+const base = import.meta.env.BASE_URL;
+const items = [
+  { href: '#concept', label: 'Concept' },
+  { href: '#pourquoi', label: 'Pourquoi' },
+  { href: '#configurations', label: 'Configurations' },
+  { href: '#architecture', label: 'Architecture' },
+  { href: '#cas-dusage', label: "Cas d'usage" },
+  { href: '#equipe', label: 'Équipe' },
+  { href: '#roadmap', label: 'Feuille de route' },
+  { href: '#contact', label: 'Contact' },
+];
+---
+
+<header class="sticky top-0 z-40 border-b border-white/5 bg-abyss-950/75 backdrop-blur">
+  <div class="container-narrow flex h-16 items-center justify-between">
+    <a href={base} class="flex items-center gap-2.5" aria-label="NowYouSea — retour à l'accueil">
+      <span class="relative inline-flex h-8 w-8 items-center justify-center rounded-md bg-gradient-to-br from-tide-500 to-lagoon-500 shadow-glow">
+        <svg viewBox="0 0 24 24" class="h-5 w-5 text-abyss-950" fill="currentColor" aria-hidden="true">
+          <path d="M2 16c2 0 2-2 4-2s2 2 4 2 2-2 4-2 2 2 4 2 2-2 4-2v4H2z"/>
+          <circle cx="12" cy="9" r="2"/>
+          <path d="M7 9a5 5 0 0 1 10 0" stroke="currentColor" stroke-width="1.5" fill="none" opacity=".65"/>
+        </svg>
+      </span>
+      <span class="font-display text-base font-semibold tracking-tight">NowYouSea</span>
+    </a>
+
+    <nav aria-label="Navigation principale" class="hidden lg:block">
+      <ul class="flex items-center gap-7 text-sm text-foam-100/75">
+        {items.map((item) => (
+          <li>
+            <a href={item.href} class="hover:text-tide-400">{item.label}</a>
+          </li>
+        ))}
+      </ul>
+    </nav>
+
+    <a href="#contact" class="btn btn-primary hidden md:inline-flex">
+      Demander une démo
+      <svg viewBox="0 0 24 24" class="h-4 w-4" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true">
+        <path d="M5 12h14M13 5l7 7-7 7"/>
+      </svg>
+    </a>
+  </div>
+</header>