fix: an application a device does not offer does not start either

Taking the launcher off the bar is half of a feature switch. The command
has to refuse as well, or a device that does not offer an application can
still be talked into running it by a stale desktop entry, a leftover
window rule or a technician's muscle memory.

Wonder Ink already did this. Parent Call now does the same, through the
same helper and with the same default: absent means yes, so every
existing room is unaffected.

Co-Authored-By: Claude Opus 5 <[email protected]>

scripts/classroom-parent-call

@@ -77,7 +77,7 @@ sys.path.insert(
 )
 
 import classroom_parent_call as contract  # noqa: E402
-from classroom_config import read_device_config  # noqa: E402
+from classroom_config import feature, read_device_config  # noqa: E402
 from classroom_mqtt import room_name  # noqa: E402
 
 APP_ID = "classroom-parent-call"
@@ -810,6 +810,15 @@ def main():
     GLib.set_prgname(APP_ID)
     GLib.set_application_name("Parent Call")
 
+    # A device that does not offer Parent Call does not offer it. The
+    # panel has no button, so nothing should reach here; saying it out
+    # loud is for the technician who ran the command by hand and wants to
+    # know why. Defaults to true, so every existing room is unaffected.
+    if not feature(read_device_config(), "PARENT_CALL_ENABLED"):
+        print("classroom-parent-call: PARENT_CALL_ENABLED is false on this "
+              "device; not starting", flush=True)
+        return 0
+
     # --child fills the name in, and --send presses the button once the
     # sanctuary has answered for whether it may be pressed. Only the lab
     # uses them: there is no teacher-facing path to either, and neither