refactor: ICMP checker type 从 ping 统一改为 icmp，修复前端 UI 细节

- ICMP checker 的 type/configKey/YAML 配置键/接口属性名从 ping 改为 icmp
- IcmpChecker 添加 platform 构造函数注入，修复 Windows 测试兼容性
- 前端 target 表格延迟列优化：标题简化为「延迟」，单位下移到单元格，宽度 80px
- Drawer 概览页 Descriptions 添加 tableLayout=auto 收窄 label 宽度
- 同步更新 README.md、DEVELOPMENT.md、probes.example.yaml、JSON Schema 和全部测试

probes.example.yaml

@@ -178,13 +178,13 @@ targets:
       banner:
         - contains: "ESMTP"
 
-  # ========== Ping targets ==========
+  # ========== ICMP targets ==========
 
-  - id: "gateway-ping"
+  - id: "gateway-icmp"
     name: "网关 ICMP 可达"
-    type: ping
+    type: icmp
     group: "基础设施"
-    ping:
+    icmp:
       host: "127.0.0.1"
       count: 3
       packetSize: 56