feat: power off monitors when lock screen activates (#1402)

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

quickshell/Common/settings/SettingsSpec.js

@@ -259,6 +259,7 @@ var SPEC = {
     lockScreenShowDate: { def: true },
     lockScreenShowProfileImage: { def: true },
     lockScreenShowPasswordField: { def: true },
+    lockScreenPowerOffMonitorsOnLock: { def: false },
     enableFprint: { def: false },
     maxFprintTries: { def: 15 },
     fprintdAvailable: { def: false, persist: false },