Update admin theme/layout and refresh README details.

Align the project baseline with the latest admin interface styling and layout structure while documenting setup and usage updates in README.

apps/api/tests/modules/admin/test_service.py

@@ -0,0 +1,33 @@
+from unittest.mock import patch
+
+from app.modules.admin.service import patch_user
+from app.modules.users import repository
+from app.modules.users.repository import create_user, get_user_by_email
+from app.core.security import hash_password
+
+
+def test_admin_cannot_self_demote():
+    admin = get_user_by_email("admin@compton.example")
+    try:
+        patch_user(admin, admin.id, "user", None)
+        assert False, "Expected self-demotion error"
+    except ValueError as exc:
+        assert str(exc) == "SELF_DEMOTION_FORBIDDEN"
+
+
+def test_admin_can_promote_user():
+    admin = get_user_by_email("admin@compton.example")
+    regular = create_user("sample@example.com", hash_password("Valid123"), role="user", status="active")
+    result = patch_user(admin, regular.id, "admin", None)
+    assert result["role"] == "admin"
+
+
+def test_last_admin_protected():
+    admin = get_user_by_email("admin@compton.example")
+    target = create_user("target@example.com", hash_password("Valid123"), role="admin", status="active")
+    with patch.object(repository, "count_admins", return_value=1):
+        try:
+            patch_user(admin, target.id, "user", None)
+            assert False, "Expected last-admin protection"
+        except ValueError as exc:
+            assert str(exc) == "LAST_ADMIN_PROTECTED"