fix: 修复集群不可达时误报100分的严重bug

- 新增 check_connectivity() 连通性预检: kubectl存在性/kubeconfig/API Server连接
- 连不上集群直接返回0分+明确错误信息+排查建议(不再静默跳过)
- 每个检查项kubectl失败时必须报critical问题(不再忽略error)
- 前端: 连接失败时显示红色错误页+错误详情+排查建议+重新诊断按钮
- 智能识别常见错误: connection refused/证书过期/超时/DNS解析失败等

frontend/src/App.vue

@@ -58,8 +58,31 @@
         </el-card>
       </div>
 
+      <!-- 集群连接失败 -->
+      <div v-if="diagnosis && !diagnosing && diagnosis.connected === false" class="conn-error-box">
+        <el-card shadow="never" class="conn-error-card">
+          <div class="conn-error-icon">
+            <el-icon :size="64" color="#f56c6c"><CircleCloseFilled /></el-icon>
+          </div>
+          <h2>无法连接到 Kubernetes 集群</h2>
+          <div class="conn-error-detail">
+            <el-alert type="error" :closable="false" show-icon>
+              <template #title>错误信息</template>
+              <pre class="conn-error-pre">{{ diagnosis.connectivity_error }}</pre>
+            </el-alert>
+            <el-alert v-if="diagnosis.connectivity_suggestion" type="warning" :closable="false" show-icon style="margin-top: 12px">
+              <template #title>排查建议</template>
+              <div>{{ diagnosis.connectivity_suggestion }}</div>
+            </el-alert>
+          </div>
+          <div class="conn-error-actions">
+            <el-button type="primary" @click="runDiagnosis" :icon="Refresh">重新诊断</el-button>
+          </div>
+        </el-card>
+      </div>
+
       <!-- 诊断结果 -->
-      <template v-if="diagnosis && !diagnosing">
+      <template v-if="diagnosis && !diagnosing && diagnosis.connected !== false">
         <!-- 概览卡片 -->
         <el-row :gutter="16" class="summary-row">
           <el-col :span="4">
@@ -219,7 +242,7 @@
 <script setup>
 import { ref, computed, nextTick } from 'vue'
 import { marked } from 'marked'
-import { Refresh, MagicStick, Loading, WarningFilled, Warning, Promotion, Monitor, CircleCheckFilled, CircleClose } from '@element-plus/icons-vue'
+import { Refresh, MagicStick, Loading, WarningFilled, Warning, Promotion, Monitor, CircleCheckFilled, CircleClose, CircleCloseFilled } from '@element-plus/icons-vue'
 import { ElMessage } from 'element-plus'
 
 const API = '/api'
@@ -405,6 +428,15 @@ body { font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', 'PingFang SC'
 /* 诊断进度 */
 .progress-box { display: flex; justify-content: center; padding-top: 40px; }
 .progress-card { width: 600px; }
+
+/* 连接失败 */
+.conn-error-box { display: flex; justify-content: center; padding-top: 40px; }
+.conn-error-card { width: 700px; text-align: center; padding: 20px 0; }
+.conn-error-icon { margin-bottom: 16px; }
+.conn-error-card h2 { font-size: 22px; color: #303133; margin-bottom: 20px; }
+.conn-error-detail { text-align: left; margin: 0 20px; }
+.conn-error-pre { white-space: pre-wrap; word-break: break-all; font-size: 13px; margin: 8px 0 0; color: #f56c6c; }
+.conn-error-actions { margin-top: 24px; }
 .progress-steps { display: flex; flex-direction: column; gap: 4px; }
 .progress-item {
   display: flex; align-items: center; gap: 10px;