fix(sxmo): re-enable automatic screen-off on idle to stop backlight battery drain

The previous commit disabled sxmo_idle.sh and the idle_locker in the
unlock/lock hooks, leaving the display backlight pinned at maximum
brightness forever (Xorg runs with -s 0 -dpms and /sys/power/state is
empty, so the panel never blanks). On a phone with no suspend this is by
far the dominant battery drain.

Restore sxmo_idle.sh and start idle_locker from the unlock and lock hooks
so the screen blanks after SXMO_UNLOCK_IDLE_TIME (default 30s) / once
locked after SXMO_LOCK_IDLE_TIME (default 8s). Locking itself stays
explicit-only, matching the documented design; only the display blanks.

Verified live on the device: after the idle timeout the state transitions
to screenoff and /sys/class/backlight/backlight/brightness drops to 0.

overlay/rootfs/usr/share/sxmo/default_hooks/sxmo_hook_lock.sh

@@ -32,7 +32,14 @@ if [ -z "$SXMO_DISABLE_LEDS" ]; then
 fi
 sxmo_jobs.sh stop periodic_wakelock_check
 
-# Automatic idle transitions are disabled; only explicit locking is used
-sxmo_jobs.sh stop idle_locker 2>/dev/null || true
+# Re-enable automatic screen-off on idle to save battery. The phone only
+# locks on explicit request; once locked we blank the screen after a short
+# idle period.
+if [ -e "$XDG_CACHE_HOME/sxmo/sxmo.noidle" ]; then
+	sxmo_jobs.sh stop idle_locker
+else
+	sxmo_jobs.sh start idle_locker sxmo_idle.sh -w \
+		timeout "${SXMO_LOCK_IDLE_TIME:-8}" 'sh -c "sxmo_state.sh set screenoff"'
+fi
 
 wait

overlay/rootfs/usr/share/sxmo/default_hooks/sxmo_hook_unlock.sh

@@ -26,7 +26,13 @@ if [ -z "$SXMO_DISABLE_LEDS" ]; then
 fi
 sxmo_jobs.sh stop periodic_wakelock_check
 
-# Automatic idle locking is disabled; only explicit locking is used
-sxmo_jobs.sh stop idle_locker 2>/dev/null || true
+# Re-enable automatic screen-off on idle to save battery. Locking stays
+# explicit (the user locks only on request); we only blank the display.
+if [ -e "$XDG_CACHE_HOME/sxmo/sxmo.noidle" ]; then
+	sxmo_jobs.sh stop idle_locker
+else
+	sxmo_jobs.sh start idle_locker sxmo_idle.sh -w \
+		timeout "${SXMO_UNLOCK_IDLE_TIME:-30}" 'sh -c "sxmo_state.sh set screenoff"'
+fi
 
 wait