refactor: use low priority workqueue for underglow and battery reporting

Blocking operations on the high priority system workqueue may result in
deadlocks, particularly when Bluetooth is in use.

app/src/rgb_underglow.c

@@ -24,6 +24,7 @@
 #include <zmk/event_manager.h>
 #include <zmk/events/activity_state_changed.h>
 #include <zmk/events/usb_conn_state_changed.h>
+#include <zmk/workqueue.h>
 
 LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
 
@@ -203,7 +204,7 @@ static void zmk_rgb_underglow_tick_handler(struct k_timer *timer) {
         return;
     }
 
-    k_work_submit(&underglow_work);
+    k_work_submit_to_queue(zmk_workqueue_lowprio_work_q(), &underglow_work);
 }
 
 K_TIMER_DEFINE(underglow_tick, zmk_rgb_underglow_tick_handler, NULL);