fix(push): 修复推送平台配置选择器及在线时间显示

- PushConfigView: 新增平台选择器（Android/iOS/HarmonyOS），加载时固定默认 ANDROID，
  避免因激活服务平台顺序不同而静默保存到错误平台
- PushManagementView: 最后在线为空时降级显示设备最后登录时间，并加注释区分 IM 在线 vs 设备登录

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tenant-platform/src/views/push/PushConfigView.vue

@@ -21,6 +21,18 @@
       style="margin-bottom:16px"
     />
 
+    <el-card style="margin-bottom:16px">
+      <template #header>目标平台</template>
+      <el-radio-group v-model="selectedPlatform" @change="applySelectedPlatformConfig">
+        <el-radio-button value="ANDROID">Android</el-radio-button>
+        <el-radio-button value="IOS">iOS</el-radio-button>
+        <el-radio-button value="HARMONY">HarmonyOS</el-radio-button>
+      </el-radio-group>
+      <el-text type="info" style="margin-left:16px;font-size:12px">
+        当前编辑：{{ selectedPlatform }} 平台的推送配置，保存时仅更新该平台
+      </el-text>
+    </el-card>
+
     <el-card style="margin-bottom:16px">
       <template #header>厂商凭据</template>
       <div class="vendor-grid">
@@ -496,11 +508,8 @@ async function loadData() {
     ])
     app.value = appRes.data.data
     services.value = svcRes.data.data
-    const activePushService = services.value.find(s => s.serviceType === 'PUSH' && s.enabled)
-      ?? services.value.find(s => s.serviceType === 'PUSH')
-    if (activePushService) {
-      selectedPlatform.value = activePushService.platform as 'ANDROID' | 'IOS' | 'HARMONY'
-    }
+    // Always default to ANDROID on load; user can switch via the platform selector
+    selectedPlatform.value = 'ANDROID'
     applySelectedPlatformConfig()
   } finally {
     loading.value = false

tenant-platform/src/views/push/PushManagementView.vue

@@ -52,7 +52,14 @@
             </el-tag>
           </el-descriptions-item>
           <el-descriptions-item label="最后在线">
-            {{ userStatus.lastSeenAt ? formatTime(userStatus.lastSeenAt) : '-' }}
+            <span v-if="userStatus.lastSeenAt">{{ formatTime(userStatus.lastSeenAt) }}</span>
+            <span v-else-if="userStatus.devices[0]?.lastLoginAt" style="color:#909399">
+              {{ formatTime(userStatus.devices[0].lastLoginAt) }}
+              <el-tooltip content="该用户未连接 IM，显示的是设备最后登录时间" placement="top">
+                <span style="font-size:11px;cursor:help">（设备登录）</span>
+              </el-tooltip>
+            </span>
+            <span v-else>-</span>
           </el-descriptions-item>
           <el-descriptions-item label="可推送设备数">
             {{ userStatus.deliverableDevices.length }}