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/FeatureCard.vue

@@ -0,0 +1,17 @@
+<script setup lang="ts">
+defineProps<{ title: string }>();
+</script>
+
+<template>
+  <div
+    class="rounded-xl border border-zinc-800 bg-zinc-900/50 p-6 transition-[transform,border-color] duration-[180ms] ease-in-out hover:-translate-y-[3px] hover:border-emerald-400/40"
+  >
+    <div
+      class="mb-4 inline-flex h-[42px] w-[42px] items-center justify-center rounded-[10px] bg-emerald-500/10 text-emerald-400"
+    >
+      <slot name="icon" />
+    </div>
+    <h3 class="m-0 mb-[7px] text-lg font-semibold text-zinc-50">{{ title }}</h3>
+    <p class="m-0 text-[14.5px] leading-[1.55] text-zinc-400"><slot /></p>
+  </div>
+</template>