Fix notification deletion bug: save {} instead of [] when list is empty

When all notifications are deleted, the persistentNotifications array
becomes empty ([]). The backend doesn't accept an empty array as a
valid value, requiring an empty object ({}) instead. This fix modifies
both index.vue and UrgentTestDialog.vue to save {} when the
notification list is empty.

Co-authored-by: Sunwuyuan <88357633+Sunwuyuan@users.noreply.github.com>

src/components/settings/SettingsExplorer.vue

@@ -1,30 +1,46 @@
 <template>
   <div class="settings-explorer">
-
-
     <div>
-      <v-text-field v-model="searchQuery" class="mb-4" clearable density="comfortable" label="搜索设置"
-                    prepend-inner-icon="mdi-magnify" variant="outlined"/>
+      <v-text-field
+        v-model="searchQuery"
+        class="mb-4"
+        clearable
+        density="comfortable"
+        label="搜索设置"
+        prepend-inner-icon="mdi-magnify"
+        variant="outlined"
+      />
 
 
       <v-list>
-        <div v-for="setting in allSettings" :key="setting.key">
-          <setting-item :key="setting.key" :disabled="setting.requireDeveloper && !isDeveloperMode"
-                        :setting-key="setting.key" @error="onSettingError"
-                        @update="onSettingUpdate"/>
-          <v-divider class="my-2"/>
+        <div
+          v-for="setting in allSettings"
+          :key="setting.key"
+        >
+          <setting-item
+            :key="setting.key"
+            :disabled="setting.requireDeveloper && !isDeveloperMode"
+            :setting-key="setting.key"
+            @error="onSettingError"
+            @update="onSettingUpdate"
+          />
+          <v-divider class="my-2" />
         </div>
       </v-list>
       <v-card border>
-        <v-card-title class="text-subtitle-1">当前配置</v-card-title>
+        <v-card-title class="text-subtitle-1">
+          当前配置
+        </v-card-title>
         <v-card-text>
           <pre class="settings-json">{{ formattedSettings }}</pre>
         </v-card-text>
         <v-card-actions>
-          <v-spacer></v-spacer>
+          <v-spacer />
           <v-btn @click="copySettingsToClipboard">
             复制到剪贴板
-            <v-icon right>mdi-content-copy</v-icon>
+            <v-icon right>
+              mdi-content-copy
+            </v-icon>
           </v-btn>
         </v-card-actions>
       </v-card>