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/env.d.ts

@@ -0,0 +1,7 @@
+/// <reference types="vite/client" />
+
+declare module '*.vue' {
+  import type { DefineComponent } from 'vue';
+  const component: DefineComponent<Record<string, unknown>, Record<string, unknown>, unknown>;
+  export default component;
+}