feat: Add packages/site — landing vitrine (Vue 3 + Tailwind 4)

- Scaffold packages/site with 11 Vue components: hero (animated terminal), features, showcase, work groups, how-it-works, security, FAQ accordion, footer
- i18n EN/FR via vue-i18n with 82+ keys, autodetect navigator.language, toggle without reload
- Animated timeline (terminal, dialog, toast) ported from design, scroll-reveal, copy buttons (2 instances)
- Self-hosted JetBrains Mono fonts (@fontsource), optimized assets (mark 28KB, og-cover 1200×630)
- SEO: canonical, og:image, theme-color, robots.txt, sitemap.xml for git-arboretum.com
- Monorepo integration (decoupled from npm release): build:site / dev:site / preview:site scripts
- CI: add build:site step to ci.yml for early detection; create .gitea/workflows/prod.yml for FTPS deploy (lftp mirror) to git-arboretum.com
- Validated builds ✓ and UI rendering (EN/FR headless test) ✓

packages/site/src/composables/useCopy.ts

@@ -0,0 +1,24 @@
+import { ref } from 'vue';
+
+export const INSTALL_COMMAND = 'npx @johanleroy/git-arboretum';
+
+// Bouton « copier la commande » : copie + bascule le label 1800 ms (fidèle au design).
+export function useCopy() {
+  const copied = ref(false);
+  let timer: ReturnType<typeof setTimeout> | undefined;
+
+  async function copy(text: string = INSTALL_COMMAND): Promise<void> {
+    try {
+      if (navigator.clipboard) await navigator.clipboard.writeText(text);
+    } catch {
+      /* clipboard indisponible (contexte non sécurisé) */
+    }
+    copied.value = true;
+    clearTimeout(timer);
+    timer = setTimeout(() => {
+      copied.value = false;
+    }, 1800);
+  }
+
+  return { copied, copy };
+}