feat: add system settings page for admin username and password

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/test_env_file.py

@@ -0,0 +1,15 @@
+from pathlib import Path
+
+from packages.config.env_file import read_env_value, update_env_file
+
+
+def test_update_env_file(tmp_path: Path):
+    envf = tmp_path / ".env"
+    envf.write_text("FOO=bar\n# comment\nBAZ=old\n", encoding="utf-8")
+    update_env_file(envf, {"BAZ": "new", "ADMIN_USERNAME": "alice"})
+    text = envf.read_text(encoding="utf-8")
+    assert "FOO=bar" in text
+    assert "BAZ=new" in text
+    assert "ADMIN_USERNAME=alice" in text
+    assert read_env_value(envf, "BAZ") == "new"
+    assert read_env_value(envf, "ADMIN_USERNAME") == "alice"