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>

tests/test_background.py

@@ -0,0 +1,105 @@
+import os
+import time
+import uuid
+
+import pytest
+from PySide6.QtCore import QCoreApplication
+
+from mtga_assistant.background import (
+    LaunchWatcher,
+    SingleInstance,
+    autostart_enabled,
+    desktop_exec,
+    launch_command,
+    set_autostart,
+)
+
+
+@pytest.fixture(scope="module")
+def app():
+    return QCoreApplication.instance() or QCoreApplication([])
+
+
+def wait_for(app, cond, timeout=3.0):
+    end = time.monotonic() + timeout
+    while time.monotonic() < end:
+        app.processEvents()
+        if cond():
+            return True
+        time.sleep(0.01)
+    app.processEvents()
+    return cond()
+
+
+def test_second_launch_messages_the_running_instance(app):
+    name = f"mtga-assistant-test-{uuid.uuid4().hex[:8]}"
+    first = SingleInstance(name)
+    assert not first.send("show")  # nobody running yet
+    assert first.listen()
+    got = []
+    first.message.connect(got.append)
+
+    second = SingleInstance(name)
+    try:
+        assert second.send("show")
+        assert wait_for(app, lambda: got == ["show"])
+    finally:
+        # Let queued socket events run before the objects go away.
+        first.close()
+        wait_for(app, lambda: False, timeout=0.2)
+
+
+def test_launch_watcher_reports_new_log_not_appends(app, tmp_path):
+    log = tmp_path / "Player.log"
+    log.write_text("old session\n")
+    watcher = LaunchWatcher(log)
+    assert watcher.active
+    launches = []
+    watcher.launched.connect(lambda: launches.append(1))
+
+    # Normal logging: no launch.
+    with open(log, "a") as f:
+        f.write("more\n")
+    wait_for(app, lambda: False, timeout=0.3)
+    assert launches == []
+
+    # MTGA starts: old log becomes Player-prev.log, a new Player.log appears.
+    os.replace(log, tmp_path / "Player-prev.log")
+    log.write_text("new session\n")
+    assert wait_for(app, lambda: len(launches) >= 1)
+
+    # Still watching the new file: a truncate-in-place restart is reported too.
+    count = len(launches)
+    with open(log, "a") as f:
+        f.write("lots of lines\n" * 50)
+    wait_for(app, lambda: False, timeout=0.3)
+    assert len(launches) == count
+    log.write_text("")
+    assert wait_for(app, lambda: len(launches) > count)
+
+
+def test_launch_watcher_without_folder_is_inactive(app, tmp_path):
+    assert not LaunchWatcher(tmp_path / "missing" / "Player.log").active
+
+
+def test_linux_autostart_entry(monkeypatch, tmp_path):
+    monkeypatch.setenv("XDG_CONFIG_HOME", str(tmp_path))
+    assert not autostart_enabled("linux")
+    set_autostart(True, "linux")
+    entry = tmp_path / "autostart" / "mtga-assistant.desktop"
+    text = entry.read_text()
+    assert autostart_enabled("linux")
+    assert "--background" in text and "Exec=" in text
+    set_autostart(False, "linux")
+    assert not entry.exists() and not autostart_enabled("linux")
+
+
+def test_launch_command_starts_hidden():
+    cmd = launch_command()
+    assert cmd[-1] == "--background"
+
+
+def test_desktop_exec_quoting():
+    assert desktop_exec(["/usr/bin/python", "-m", "x"]) == "/usr/bin/python -m x"
+    assert desktop_exec(["/a b/c"]) == '"/a b/c"'
+    assert desktop_exec(['/a"b']) == '"/a\\"b"'