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/ProblemSection.vue

@@ -0,0 +1,14 @@
+<script setup lang="ts">
+import { useI18n } from 'vue-i18n';
+const { t } = useI18n();
+</script>
+
+<template>
+  <section v-reveal class="mx-auto max-w-[920px] px-6 pb-[92px] pt-12 text-center">
+    <div class="mb-4 font-mono text-xs uppercase tracking-[0.14em] text-emerald-400">{{ t('probKicker') }}</div>
+    <h2 class="m-0 text-[clamp(28px,3.6vw,42px)] font-semibold leading-[1.18] tracking-[-0.025em] text-zinc-50">
+      {{ t('probTitle') }}
+    </h2>
+    <p class="mx-auto mt-[22px] max-w-[640px] text-[clamp(17px,1.6vw,19px)] text-zinc-400">{{ t('probBody') }}</p>
+  </section>
+</template>