feat: Establish core application layout, global styles, and theme mode management.

Frontend-Learner/layouts/default.vue

@@ -10,6 +10,9 @@ const leftDrawerOpen = ref(false)
 const toggleLeftDrawer = () => {
   leftDrawerOpen.value = !leftDrawerOpen.value
 }
+
+// Initialize global theme management
+useThemeMode()
 </script>
 
 <template>