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/app/api/auth/logout/route.ts

@@ -0,0 +1,8 @@
+import { NextResponse } from "next/server";
+import { SESSION_COOKIE } from "@/lib/session";
+
+export async function POST() {
+  const response = NextResponse.json({ success: true });
+  response.cookies.delete(SESSION_COOKIE);
+  return response;
+}