Add idle_task_init() call in main()

The idle task is now initialized in main() during system startup.
This ensures that the scheduler always has a valid execution context
before any user or application tasks are created. Initializing the
idle task early guarantees a safe fallback path when no runnable
tasks exist and keeps the scheduler entry point consistent.

Author: vicLin8712

kernel/main.c

@@ -23,6 +23,9 @@ int32_t main(void)
     printf("Heap initialized, %u bytes available\n",
            (unsigned int) (size_t) &_heap_size);
 
+    /* Initialize idle task */
+    idle_task_init();
+
     /* Call the application's main entry point to create initial tasks. */
     kcb->preemptive = (bool) app_main();
     printf("Scheduler mode: %s\n",