Screen: switch off by itself when nothing is going on

With `general.screen.off_command` set, the monitor switches off once nothing has
played and nobody has done anything for `auto_off_minutes` (15 by default, 0 for
never, editable in parent mode). "Doing anything" is any intent a person could
have caused: a remote key, a web command, or a key press or tap in the kiosk,
which the page reports at most every 30s via POST /api/screen/activity. The first
such intent after an auto-off runs `on_command`, so the remote wakes the screen
too. The auto-off itself is silent; switches a person asks for still blubb.

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

python-backend/tests/test_config.py

@@ -114,6 +114,14 @@ def test_a_series_slot_cannot_name_a_song(config_dir: Path) -> None:
     assert "track_index" in message
 
 
+def test_the_screen_switches_off_after_15_minutes_by_default(config_dir: Path) -> None:
+    data = _config(screen={"toggle_command": "kiosk-screen toggle"})
+    screen = load_config(write_config(config_dir, data)).general.screen
+    assert screen is not None
+    assert screen.auto_off_minutes == 15
+    assert screen.off_command is None  # ...but only once there is a way to do it
+
+
 def test_ha_device_and_scene_name_is_optional(config_dir: Path) -> None:
     data = _config(
         ha={