feat: add user avatar and username, match moonlock icon style

- Add users.py with avatar detection (pwd, AccountsService, ~/.face)
- Display avatar + username above action buttons
- Look up 22px icon variant (same as moonlock) and render at 64px
- Round action buttons (border-radius 50%)
- 9 new tests for user/avatar detection (63 total)

tests/test_users.py

@@ -0,0 +1,95 @@
+# ABOUTME: Tests for current user detection and avatar loading.
+# ABOUTME: Verifies user info retrieval from the system.
+
+from pathlib import Path
+from unittest.mock import patch
+
+from moonset.users import get_current_user, get_avatar_path, get_default_avatar_path, User
+
+
+class TestGetCurrentUser:
+    """Tests for current user detection."""
+
+    @patch("moonset.users.os.getuid", return_value=1000)
+    @patch("moonset.users.pwd.getpwuid")
+    def test_returns_user_with_correct_username(self, mock_pwd, mock_uid):
+        mock_pwd.return_value.pw_name = "testuser"
+        mock_pwd.return_value.pw_gecos = "Test User"
+        mock_pwd.return_value.pw_dir = "/home/testuser"
+        mock_pwd.return_value.pw_uid = 1000
+        user = get_current_user()
+        assert user.username == "testuser"
+        assert user.display_name == "Test User"
+        assert user.home == Path("/home/testuser")
+        mock_pwd.assert_called_once_with(1000)
+
+    @patch("moonset.users.os.getuid", return_value=1000)
+    @patch("moonset.users.pwd.getpwuid")
+    def test_empty_gecos_falls_back_to_username(self, mock_pwd, mock_uid):
+        mock_pwd.return_value.pw_name = "testuser"
+        mock_pwd.return_value.pw_gecos = ""
+        mock_pwd.return_value.pw_dir = "/home/testuser"
+        mock_pwd.return_value.pw_uid = 1000
+        user = get_current_user()
+        assert user.display_name == "testuser"
+
+    @patch("moonset.users.os.getuid", return_value=1000)
+    @patch("moonset.users.pwd.getpwuid")
+    def test_gecos_with_commas_uses_first_field(self, mock_pwd, mock_uid):
+        mock_pwd.return_value.pw_name = "testuser"
+        mock_pwd.return_value.pw_gecos = "Test User,,,Room 42"
+        mock_pwd.return_value.pw_dir = "/home/testuser"
+        mock_pwd.return_value.pw_uid = 1000
+        user = get_current_user()
+        assert user.display_name == "Test User"
+
+
+class TestGetAvatarPath:
+    """Tests for avatar path resolution."""
+
+    def test_returns_face_file_if_exists(self, tmp_path: Path):
+        face = tmp_path / ".face"
+        face.write_text("fake image")
+        path = get_avatar_path(tmp_path)
+        assert path == face
+
+    def test_returns_accountsservice_icon_if_exists(self, tmp_path: Path):
+        username = "testuser"
+        icons_dir = tmp_path / "icons"
+        icons_dir.mkdir()
+        icon = icons_dir / username
+        icon.write_text("fake image")
+        path = get_avatar_path(
+            tmp_path, username=username, accountsservice_dir=icons_dir
+        )
+        assert path == icon
+
+    def test_face_file_takes_priority_over_accountsservice(self, tmp_path: Path):
+        face = tmp_path / ".face"
+        face.write_text("fake image")
+        icons_dir = tmp_path / "icons"
+        icons_dir.mkdir()
+        icon = icons_dir / "testuser"
+        icon.write_text("fake image")
+        path = get_avatar_path(
+            tmp_path, username="testuser", accountsservice_dir=icons_dir
+        )
+        assert path == face
+
+    def test_returns_none_when_no_avatar(self, tmp_path: Path):
+        path = get_avatar_path(tmp_path)
+        assert path is None
+
+
+class TestGetDefaultAvatarPath:
+    """Tests for default avatar fallback."""
+
+    def test_default_avatar_exists(self):
+        """The package default avatar must always be present."""
+        path = get_default_avatar_path()
+        assert path.is_file()
+
+    def test_default_avatar_is_svg(self):
+        """The default avatar should be an SVG file."""
+        path = get_default_avatar_path()
+        assert path.suffix == ".svg"