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

probe-config.schema.json

@@ -84,7 +84,7 @@
             }
           }
         },
-        "ping": {
+        "icmp": {
           "additionalProperties": false,
           "type": "object",
           "properties": {}
@@ -2479,7 +2479,7 @@
             "required": [
               "id",
               "type",
-              "ping"
+              "icmp"
             ],
             "properties": {
               "description": {
@@ -2829,10 +2829,10 @@
                 "type": "string"
               },
               "type": {
-                "const": "ping",
+                "const": "icmp",
                 "type": "string"
               },
-              "ping": {
+              "icmp": {
                 "additionalProperties": false,
                 "type": "object",
                 "required": [