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) ✓

package.json

@@ -15,7 +15,10 @@
     "pack": "npm run build && npm pack -w @johanleroy/git-arboretum",
     "test": "vitest run",
     "dev:server": "npm run dev -w @johanleroy/git-arboretum",
-    "dev:web": "npm run dev -w @arboretum/web"
+    "dev:web": "npm run dev -w @arboretum/web",
+    "build:site": "npm run build -w @arboretum/site",
+    "dev:site": "npm run dev -w @arboretum/site",
+    "preview:site": "npm run preview -w @arboretum/site"
   },
   "devDependencies": {
     "@types/node": "^22.10.0",