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>

.env.example

@@ -13,6 +13,12 @@
 # Use this token for the first API calls before creating proper API tokens
 API_BOOTSTRAP_TOKEN=change-me-in-production
 
+# Backoffice admin credentials (required)
+ADMIN_USERNAME=admin
+ADMIN_PASSWORD=change-me-in-production
+# Secret for signing session JWTs (min 32 chars, required)
+SESSION_SECRET=change-me-in-production-use-32-chars-min
+
 # =============================================================================
 # Service Configuration
 # =============================================================================