feat: add backoffice authentication with login page

- Add login page with logo background, glassmorphism card
- Add session management via JWT (jose) with httpOnly cookie
- Add Next.js proxy middleware to protect all routes
- Add logout button in nav
- Restructure app into (app) route group to isolate login layout
- Add ADMIN_USERNAME, ADMIN_PASSWORD, SESSION_SECRET env vars

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/backoffice/package.json

@@ -8,6 +8,7 @@
     "start": "next start -p 7082"
   },
   "dependencies": {
+    "jose": "^6.2.2",
     "next": "^16.1.6",
     "next-themes": "^0.4.6",
     "react": "19.0.0",