feat: UI-Polish und Konfiguration

Wallpaper-Support via moongreet.toml, Astronauten-SVG als Default-Avatar
mit dynamischer Theme-Farbe, CSS auf GTK-Theme-Farben umgestellt,
konsistentes Widget-Spacing, Custom-Icon-Registrierung.

tests/test_config.py

@@ -0,0 +1,47 @@
+# ABOUTME: Tests for configuration loading from moongreet.toml.
+# ABOUTME: Verifies parsing of appearance and behavior settings.
+
+from pathlib import Path
+
+import pytest
+
+from moongreet.config import load_config, Config
+
+
+class TestLoadConfig:
+    """Tests for loading moongreet.toml configuration."""
+
+    def test_loads_background_path(self, tmp_path: Path) -> None:
+        toml_file = tmp_path / "moongreet.toml"
+        toml_file.write_text(
+            "[appearance]\n"
+            'background = "/usr/share/backgrounds/test.jpg"\n'
+        )
+
+        config = load_config(toml_file)
+
+        assert config.background == Path("/usr/share/backgrounds/test.jpg")
+
+    def test_returns_none_background_when_missing(self, tmp_path: Path) -> None:
+        toml_file = tmp_path / "moongreet.toml"
+        toml_file.write_text("[appearance]\n")
+
+        config = load_config(toml_file)
+
+        assert config.background is None
+
+    def test_returns_defaults_for_missing_file(self, tmp_path: Path) -> None:
+        config = load_config(tmp_path / "nonexistent.toml")
+
+        assert config.background is None
+
+    def test_resolves_relative_path_against_config_dir(self, tmp_path: Path) -> None:
+        toml_file = tmp_path / "moongreet.toml"
+        toml_file.write_text(
+            "[appearance]\n"
+            'background = "wallpaper.jpg"\n'
+        )
+
+        config = load_config(toml_file)
+
+        assert config.background == tmp_path / "wallpaper.jpg"