Run in the background and open when MTGA starts

- Closing the window hides it to the system tray; tracking and history
  recording continue. Quit from the tray, the menu or Ctrl+Q.
- Show the window when MTGA starts a new Player.log, detected with OS
  file notifications on the log folder (QFileSystemWatcher), not polling.
- Single instance over a local socket: launching again shows the
  running window (or stays hidden with --background).
- Menu options for background mode, show-on-launch and start at login
  (XDG autostart on Linux, HKCU Run key on Windows); --background flag.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/mtga_assistant/app.py

@@ -1,4 +1,4 @@
-"""Entry point: mtga-assistant [--log PATH] [--replay PATH [--rate N]]."""
+"""Entry point: mtga-assistant [--background] [--log PATH] [--replay PATH [--rate N]]."""
 
 from __future__ import annotations
 
@@ -10,6 +10,7 @@ from PySide6.QtGui import QIcon
 from PySide6.QtWidgets import QApplication
 
 from . import paths
+from .background import SingleInstance
 from .carddb import open_card_db
 from .history import HistoryStore
 from .images import ImageCache
@@ -23,6 +24,8 @@ def main() -> int:
     ap.add_argument("--replay", type=Path, help="replay a saved log instead of following the live one")
     ap.add_argument("--rate", type=float, default=40.0, help="replay speed in log events per second")
     ap.add_argument("--mtga-dir", type=Path, help="MTGA install directory (default: auto-detect)")
+    ap.add_argument("--background", action="store_true",
+                    help="start hidden (e.g. at login); the window opens when MTGA starts")
     args, qt_args = ap.parse_known_args()
 
     cfg = paths.load_config()
@@ -37,6 +40,15 @@ def main() -> int:
 
         ctypes.windll.shell32.SetCurrentProcessExplicitAppUserModelID("mtga-assistant")
     app = QApplication([sys.argv[0], *qt_args])
+    app.setQuitOnLastWindowClosed(False)  # closing the window keeps recording in the background
+
+    # One instance per user: a second launch just asks the running one to show its window.
+    instance = None
+    if not args.replay:
+        instance = SingleInstance()
+        if instance.send("background" if args.background else "show"):
+            return 0
+        instance.listen()
     app.setWindowIcon(QIcon(str(Path(__file__).with_name("icon.png"))))
     app.setApplicationName("MTGA Assistant")
     app.setDesktopFileName("mtga-assistant")  # Wayland app_id, usable in Hyprland window rules
@@ -57,7 +69,10 @@ def main() -> int:
             # Matches from the previous MTGA session are only in Player-prev.log.
             win.import_history(log_path.with_name("Player-prev.log"))
     app.aboutToQuit.connect(win.shutdown)
-    win.show()
+    if instance is not None:
+        instance.message.connect(lambda msg: win.bring_to_front() if msg == "show" else None)
+    if not args.background:
+        win.show()
     return app.exec()