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/next-env.d.ts

@@ -1,6 +1,6 @@
 /// <reference types="next" />
 /// <reference types="next/image-types/global" />
-import "./.next/types/routes.d.ts";
+import "./.next/dev/types/routes.d.ts";
 
 // NOTE: This file should not be edited
 // see https://nextjs.org/docs/app/api-reference/config/typescript for more information.