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/LangToggle.vue

@@ -0,0 +1,37 @@
+<script setup lang="ts">
+import { useI18n } from 'vue-i18n';
+import { setLocale, type AppLocale } from '../i18n';
+
+const { locale } = useI18n();
+
+function set(l: AppLocale): void {
+  setLocale(l);
+}
+</script>
+
+<template>
+  <div
+    role="group"
+    aria-label="Language"
+    class="flex items-center overflow-hidden rounded-lg border border-zinc-800 bg-zinc-950/40"
+  >
+    <button
+      type="button"
+      :aria-pressed="locale === 'en'"
+      class="cursor-pointer border-none px-[11px] py-1.5 font-mono text-xs font-semibold focus-visible:outline-2 focus-visible:outline-offset-1 focus-visible:outline-emerald-500/70"
+      :class="locale === 'en' ? 'bg-zinc-800 text-emerald-400' : 'bg-transparent text-zinc-500'"
+      @click="set('en')"
+    >
+      EN
+    </button>
+    <button
+      type="button"
+      :aria-pressed="locale === 'fr'"
+      class="cursor-pointer border-none px-[11px] py-1.5 font-mono text-xs font-semibold focus-visible:outline-2 focus-visible:outline-offset-1 focus-visible:outline-emerald-500/70"
+      :class="locale === 'fr' ? 'bg-zinc-800 text-emerald-400' : 'bg-transparent text-zinc-500'"
+      @click="set('fr')"
+    >
+      FR
+    </button>
+  </div>
+</template>