feat: Multi-Monitor-Support — Wallpaper auf Sekundärmonitoren

- WallpaperWindow für Sekundärmonitore (nur Hintergrundbild)
- GreeterWindow bekommt bg_path als Parameter
- resolve_wallpaper_path() aus config.py extrahiert (wiederverwendbar)
- main.py: Monitor-Enumeration, Layer-Shell pro Monitor
- Keyboard-Exclusive nur auf dem primären Monitor
- CSS: ungültige max-width/max-height Properties entfernt

tests/test_config.py

@@ -5,7 +5,7 @@ from pathlib import Path
 
 import pytest
 
-from moongreet.config import load_config, Config
+from moongreet.config import load_config, resolve_wallpaper_path, Config
 
 
 class TestLoadConfig:
@@ -53,3 +53,38 @@ class TestLoadConfig:
         config = load_config(toml_file)
 
         assert config.background == tmp_path / "wallpaper.jpg"
+
+
+class TestResolveWallpaperPath:
+    """Tests for resolving the wallpaper path from config or package default."""
+
+    def test_uses_configured_path_when_exists(self, tmp_path: Path) -> None:
+        wallpaper = tmp_path / "custom.jpg"
+        wallpaper.write_bytes(b"fake-image")
+        config = Config(background=wallpaper)
+
+        path, ctx = resolve_wallpaper_path(config)
+
+        assert path == wallpaper
+        assert ctx is None
+
+    def test_falls_back_to_package_default(self) -> None:
+        config = Config(background=None)
+
+        path, ctx = resolve_wallpaper_path(config)
+
+        assert path is not None
+        assert path.exists()
+        assert ctx is not None
+        # Clean up context manager
+        ctx.__exit__(None, None, None)
+
+    def test_falls_back_when_configured_path_missing(self, tmp_path: Path) -> None:
+        config = Config(background=tmp_path / "nonexistent.jpg")
+
+        path, ctx = resolve_wallpaper_path(config)
+
+        assert path is not None
+        assert path.exists()
+        assert ctx is not None
+        ctx.__exit__(None, None, None)