fix: Display-Null-Check und File-Logging in main.py

Gdk.Display.get_default() kann None zurueckgeben wenn der Compositor
noch nicht bereit ist. Vorher crashte moongreet mit TypeError, ohne
dass der Fehler irgendwo geloggt wurde. Display wird jetzt einmal
geholt, geprueft und an _register_icons/_load_css durchgereicht.
Logging geht nach /var/cache/moongreet/moongreet.log und stderr.

src/moongreet/main.py

@@ -4,6 +4,7 @@
 import logging
 import sys
 from importlib.resources import files
+from pathlib import Path
 
 import gi
 gi.require_version("Gtk", "4.0")
@@ -21,9 +22,38 @@ try:
 except (ValueError, ImportError):
     HAS_LAYER_SHELL = False
 
+LOG_DIR = Path("/var/cache/moongreet")
+LOG_FILE = LOG_DIR / "moongreet.log"
+
 logger = logging.getLogger(__name__)
 
 
+def _setup_logging() -> None:
+    """Configure logging to file and stderr."""
+    root = logging.getLogger()
+    root.setLevel(logging.DEBUG)
+
+    formatter = logging.Formatter(
+        "%(asctime)s %(levelname)s %(name)s: %(message)s"
+    )
+
+    # Always log to stderr
+    stderr_handler = logging.StreamHandler(sys.stderr)
+    stderr_handler.setLevel(logging.DEBUG)
+    stderr_handler.setFormatter(formatter)
+    root.addHandler(stderr_handler)
+
+    # Log to file if the directory is writable
+    if LOG_DIR.is_dir():
+        try:
+            file_handler = logging.FileHandler(LOG_FILE)
+            file_handler.setLevel(logging.DEBUG)
+            file_handler.setFormatter(formatter)
+            root.addHandler(file_handler)
+        except PermissionError:
+            logger.warning("Cannot write to %s", LOG_FILE)
+
+
 class MoongreetApp(Gtk.Application):
     """GTK Application for the Moongreet greeter."""
 
@@ -34,14 +64,18 @@ class MoongreetApp(Gtk.Application):
 
     def do_activate(self) -> None:
         """Create and present greeter windows on all monitors."""
-        self._register_icons()
-        self._load_css()
+        display = Gdk.Display.get_default()
+        if display is None:
+            logger.error("No display available — cannot start greeter UI")
+            return
+
+        self._register_icons(display)
+        self._load_css(display)
 
         # Resolve wallpaper once, share across all windows
         config = load_config()
         bg_path, self._wallpaper_ctx = resolve_wallpaper_path(config)
 
-        display = Gdk.Display.get_default()
         monitors = display.get_monitors()
         primary_monitor = None
 
@@ -81,19 +115,19 @@ class MoongreetApp(Gtk.Application):
             self._wallpaper_ctx = None
         Gtk.Application.do_shutdown(self)
 
-    def _register_icons(self) -> None:
+    def _register_icons(self, display: Gdk.Display) -> None:
         """Register custom icons from the package data/icons directory."""
         icons_dir = files("moongreet") / "data" / "icons"
-        icon_theme = Gtk.IconTheme.get_for_display(Gdk.Display.get_default())
+        icon_theme = Gtk.IconTheme.get_for_display(display)
         icon_theme.add_search_path(str(icons_dir))
 
-    def _load_css(self) -> None:
+    def _load_css(self, display: Gdk.Display) -> None:
         """Load the CSS stylesheet for the greeter."""
         css_provider = Gtk.CssProvider()
         css_path = files("moongreet") / "style.css"
         css_provider.load_from_path(str(css_path))
         Gtk.StyleContext.add_provider_for_display(
-            Gdk.Display.get_default(),
+            display,
             css_provider,
             Gtk.STYLE_PROVIDER_PRIORITY_APPLICATION,
         )
@@ -118,6 +152,8 @@ class MoongreetApp(Gtk.Application):
 
 def main() -> None:
     """Run the Moongreet application."""
+    _setup_logging()
+    logger.info("Moongreet starting")
     app = MoongreetApp()
     app.run(sys.argv)