feat: initial Moongreet greeter implementation

greetd-Greeter für Wayland mit Python + GTK4 + gtk4-layer-shell.
Enthält IPC-Protokoll, User/Session-Erkennung, Power-Actions,
komplettes UI-Layout und 36 Tests (Unit + Integration).

src/moongreet/sessions.py

@@ -0,0 +1,62 @@
+# ABOUTME: Session detection — discovers available Wayland and X11 sessions.
+# ABOUTME: Parses .desktop files from standard session directories.
+
+import configparser
+from dataclasses import dataclass
+from pathlib import Path
+
+DEFAULT_WAYLAND_DIRS = [Path("/usr/share/wayland-sessions")]
+DEFAULT_XSESSION_DIRS = [Path("/usr/share/xsessions")]
+
+
+@dataclass
+class Session:
+    """Represents an available login session."""
+
+    name: str
+    exec_cmd: str
+    session_type: str  # "wayland" or "x11"
+
+
+def _parse_desktop_file(path: Path, session_type: str) -> Session | None:
+    """Parse a .desktop file and return a Session, or None if invalid."""
+    config = configparser.ConfigParser(interpolation=None)
+    config.read(path)
+
+    section = "Desktop Entry"
+    if not config.has_section(section):
+        return None
+
+    name = config.get(section, "Name", fallback=None)
+    exec_cmd = config.get(section, "Exec", fallback=None)
+
+    if not name or not exec_cmd:
+        return None
+
+    return Session(name=name, exec_cmd=exec_cmd, session_type=session_type)
+
+
+def get_sessions(
+    wayland_dirs: list[Path] = DEFAULT_WAYLAND_DIRS,
+    xsession_dirs: list[Path] = DEFAULT_XSESSION_DIRS,
+) -> list[Session]:
+    """Discover available sessions from .desktop files."""
+    sessions: list[Session] = []
+
+    for directory in wayland_dirs:
+        if not directory.exists():
+            continue
+        for desktop_file in sorted(directory.glob("*.desktop")):
+            session = _parse_desktop_file(desktop_file, "wayland")
+            if session:
+                sessions.append(session)
+
+    for directory in xsession_dirs:
+        if not directory.exists():
+            continue
+        for desktop_file in sorted(directory.glob("*.desktop")):
+            session = _parse_desktop_file(desktop_file, "x11")
+            if session:
+                sessions.append(session)
+
+    return sessions