fix(frontend/LedDebug): 修复初始选中设备未正确初始化的问题

将selectedDevice初始值从1改为undefined，避免无终端设备时出现无效默认选中值，同时调整赋值逻辑的代码格式

frontend/src/views/LedDebug.vue

@@ -147,7 +147,7 @@ import { useDataStore } from '../stores/dataStore'
 
 const dataStore = useDataStore()
 
-const selectedDevice = ref(1)
+const selectedDevice = ref(undefined)
 const selectedLed = ref(1)
 const terminals = ref([])
 const loading = ref(-1)
@@ -225,7 +225,9 @@ async function loadTerminals() {
       terminals.value = entries.map(([uid, addr]) => ({
         uid, uid_readable: uid.match(/.{1,2}/g)?.join(':') || uid, address: addr
       }))
-      if (terminals.value.length) selectedDevice.value = terminals.value[0].address
+      if (terminals.value.length) {
+        selectedDevice.value = terminals.value[0].address
+      }
     }
   } catch (e) { console.error(e) }
 }