Add ability to immediately power off monitors when the lock screen
activates, controlled by a new setting "Power off monitors on lock".
Uses a 100ms polling timer to detect when the session lock actually
becomes active, then invokes compositor-specific DPMS commands.
For niri, uses the new power-off-monitors action via niri msg CLI
with socket fallback.
Wake on input: first input after lock arms wake, second input
actually powers monitors back on while keeping the lock screen visible.
Closes#1157
