feat: restore portable layout on shutdown and before suspend

Add WriteConfig/Prepare backend methods (real for Hyprland, no-op for
Sway), -apply/-prepare/-no-reload CLI flags, ExecStartPre cleanup and
suspend systemd hooks so a stale docked config can never leave the
built-in display disabled after resume.

internal/backend/interface.go

@@ -96,6 +96,18 @@ type Backend interface {
 	// are enabled is ever visible.
 	ApplyLayout(ctx context.Context, monitors []MonitorConfig) error
 
+	// WriteConfig persists the monitor configuration to disk without
+	// reloading the compositor. Used during shutdown (SIGTERM) when
+	// the compositor may not be available. For backends that don't
+	// use a config file (e.g. Sway), this is a no-op.
+	WriteConfig(ctx context.Context, monitors []MonitorConfig) error
+
+	// Prepare ensures the monitor config file has no disabled entries.
+	// This prevents the "no active displays" issue when the compositor
+	// starts with a stale config. For backends that don't use a config
+	// file (e.g. Sway), this is a no-op.
+	Prepare(ctx context.Context) error
+
 	// Events returns a channel of hotplug events and a channel of errors.
 	// The caller must read from both channels. When ctx is cancelled,
 	// the backend closes both channels and stops listening.