Add wallpaper and default avatar support

Wallpaper with fallback hierarchy: config path > Moonarch system
default (/usr/share/moonarch/wallpaper.jpg) > package fallback.
Applied to both primary and secondary monitors.

Default avatar from Moongreet ecosystem with theme-colored SVG
rendering via PixbufLoader and proper clipping frame (Gtk.Box
with overflow hidden), matching the Moongreet avatar pattern.

tests/test_users.py

@@ -5,7 +5,7 @@ import os
 from pathlib import Path
 from unittest.mock import patch
 
-from moonlock.users import get_current_user, get_avatar_path, User
+from moonlock.users import get_current_user, get_avatar_path, get_default_avatar_path, User
 
 
 class TestGetCurrentUser:
@@ -79,3 +79,17 @@ class TestGetAvatarPath:
     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"

tests/test_wallpaper.py

@@ -0,0 +1,53 @@
+# ABOUTME: Tests for wallpaper path resolution.
+# ABOUTME: Verifies fallback hierarchy: config > Moonarch system default > package fallback.
+
+from pathlib import Path
+from unittest.mock import patch
+
+from moonlock.config import Config, resolve_background_path, MOONARCH_WALLPAPER, PACKAGE_WALLPAPER
+
+
+class TestResolveBackgroundPath:
+    """Tests for the wallpaper fallback hierarchy."""
+
+    def test_config_path_used_when_file_exists(self, tmp_path: Path):
+        """Config background_path takes priority if the file exists."""
+        wallpaper = tmp_path / "custom.jpg"
+        wallpaper.write_bytes(b"\xff\xd8")
+        config = Config(background_path=str(wallpaper))
+        result = resolve_background_path(config)
+        assert result == wallpaper
+
+    def test_config_path_skipped_when_file_missing(self, tmp_path: Path):
+        """Config path should be skipped if the file does not exist."""
+        config = Config(background_path="/nonexistent/wallpaper.jpg")
+        with patch("moonlock.config.MOONARCH_WALLPAPER", tmp_path / "nope.jpg"):
+            result = resolve_background_path(config)
+        assert result == PACKAGE_WALLPAPER
+
+    def test_moonarch_default_used_when_no_config(self, tmp_path: Path):
+        """Moonarch system wallpaper is used when config has no background_path."""
+        moonarch_wp = tmp_path / "wallpaper.jpg"
+        moonarch_wp.write_bytes(b"\xff\xd8")
+        config = Config(background_path=None)
+        with patch("moonlock.config.MOONARCH_WALLPAPER", moonarch_wp):
+            result = resolve_background_path(config)
+        assert result == moonarch_wp
+
+    def test_moonarch_default_skipped_when_missing(self, tmp_path: Path):
+        """Falls back to package wallpaper when Moonarch default is missing."""
+        config = Config(background_path=None)
+        with patch("moonlock.config.MOONARCH_WALLPAPER", tmp_path / "nope.jpg"):
+            result = resolve_background_path(config)
+        assert result == PACKAGE_WALLPAPER
+
+    def test_package_fallback_always_exists(self):
+        """The package fallback wallpaper must always be present."""
+        assert PACKAGE_WALLPAPER.is_file()
+
+    def test_full_fallback_chain(self, tmp_path: Path):
+        """With no config and no Moonarch default, package fallback is returned."""
+        config = Config()
+        with patch("moonlock.config.MOONARCH_WALLPAPER", tmp_path / "nope.jpg"):
+            result = resolve_background_path(config)
+        assert result == PACKAGE_WALLPAPER