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/components/LogoutButton.tsx

@@ -0,0 +1,27 @@
+"use client";
+
+import { useRouter } from "next/navigation";
+
+export function LogoutButton() {
+  const router = useRouter();
+
+  async function handleLogout() {
+    await fetch("/api/auth/logout", { method: "POST" });
+    router.push("/login");
+    router.refresh();
+  }
+
+  return (
+    <button
+      onClick={handleLogout}
+      title="Se déconnecter"
+      className="p-2 rounded-lg text-muted-foreground hover:text-foreground hover:bg-accent transition-colors"
+    >
+      <svg xmlns="http://www.w3.org/2000/svg" width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
+        <path d="M9 21H5a2 2 0 0 1-2-2V5a2 2 0 0 1 2-2h4" />
+        <polyline points="16 17 21 12 16 7" />
+        <line x1="21" y1="12" x2="9" y2="12" />
+      </svg>
+    </button>
+  );
+}