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/components/AppHeader.vue

@@ -0,0 +1,59 @@
+<script setup lang="ts">
+import { useI18n } from 'vue-i18n';
+import LangToggle from './LangToggle.vue';
+import IconGitea from './icons/IconGitea.vue';
+
+const { t } = useI18n();
+const REPO = 'https://git.lidge.fr/johanleroy/git-arboretum';
+
+const navLinks = [
+  { href: '#features', key: 'navFeatures' },
+  { href: '#how', key: 'navHow' },
+  { href: '#security', key: 'navSecurity' },
+  { href: '#faq', key: 'navFaq' },
+] as const;
+</script>
+
+<template>
+  <header
+    class="sticky top-0 z-50 border-b border-[#1c1c1f] bg-[rgba(9,9,11,0.72)] backdrop-blur-[14px]"
+  >
+    <div class="mx-auto flex h-16 max-w-[1200px] items-center justify-between gap-6 px-6">
+      <a href="#top" class="flex items-center gap-2.5 text-zinc-100 no-underline">
+        <img
+          src="/assets/arboretum-mark.png"
+          alt="Arboretum"
+          width="28"
+          height="28"
+          class="block h-7 w-7 object-contain"
+        />
+        <span class="font-mono text-[17px] font-semibold tracking-[-0.01em]">Arboretum</span>
+      </a>
+
+      <nav class="hidden items-center gap-[30px] min-[900px]:flex">
+        <a
+          v-for="link in navLinks"
+          :key="link.href"
+          :href="link.href"
+          class="text-[14.5px] text-zinc-400 no-underline transition-colors hover:text-zinc-100"
+        >
+          {{ t(link.key) }}
+        </a>
+      </nav>
+
+      <div class="flex items-center gap-3.5">
+        <LangToggle />
+        <a
+          :href="REPO"
+          target="_blank"
+          rel="noopener"
+          aria-label="Gitea"
+          class="inline-flex items-center gap-[7px] rounded-lg border border-zinc-800 px-[13px] py-[7px] text-[13.5px] font-medium text-zinc-300 no-underline transition-colors hover:border-emerald-500/50 hover:text-emerald-400 focus-visible:outline-2 focus-visible:outline-offset-2 focus-visible:outline-emerald-500/70"
+        >
+          <IconGitea :size="16" />
+          Gitea
+        </a>
+      </div>
+    </div>
+  </header>
+</template>