feat(i18n): AlertCenter 告警中心国际化 — 38 处硬编码中文改翻译键 + alert 命名空间(58 键)

- AlertCenter.tsx: StatStrip 4 标签/5 个 Tab 标题/搜索/级别+状态筛选/表头 6 列/分页/加载空状态/toast 12 处/确认弹框 3 处/一键报警描述/规则启用禁用/联系人联系组增删 全量 t() 化
- 复用 common.* 通用键(refresh/edit/delete/status)
- 新增 alert.* 命名空间: stat/tab/severity/status/table 子组 + searchPlaceholder/quickSetup/toast 等 58 键
- zh-CN/en-US 键总数 285→343 (alert 58 键, 中英完全对齐)
- tsc -b=0, npm build=✓(19.59s), 17/17 集成测试通过

frontend/src/locales/en-US.json

@@ -317,5 +317,77 @@
       "status": "Status",
       "tags": "Tags"
     }
+  },
+  "alert": {
+    "stat": {
+      "firing": "Firing",
+      "critical": "Critical",
+      "warning": "Warning",
+      "resolved": "Resolved"
+    },
+    "tab": {
+      "alerts": "Alerts",
+      "rules": "Alert Rules",
+      "silences": "Silence Rules",
+      "contacts": "Contacts",
+      "groups": "Contact Groups"
+    },
+    "searchPlaceholder": "Search alerts...",
+    "allSeverity": "All Severity",
+    "allStatus": "All Status",
+    "severity": {
+      "critical": "Critical",
+      "warning": "Warning",
+      "info": "Info"
+    },
+    "status": {
+      "firing": "Firing",
+      "acknowledged": "Acknowledged",
+      "resolved": "Resolved"
+    },
+    "table": {
+      "alertInfo": "Alert Info",
+      "severity": "Severity",
+      "status": "Status",
+      "source": "Source",
+      "duration": "Duration",
+      "actions": "Actions"
+    },
+    "loading": "Loading...",
+    "noAlerts": "No alerts",
+    "totalItems": "{{count}} total",
+    "acknowledged": "Alert acknowledged",
+    "resolvedToast": "Alert resolved",
+    "operationFailed": "Operation failed",
+    "ruleUpdated": "Rule status updated",
+    "updateFailed": "Update failed",
+    "deleted": "Deleted",
+    "deleteFailed": "Delete failed",
+    "deleteRule": "Delete Rule",
+    "deleteContact": "Delete Contact",
+    "deleteGroup": "Delete Group",
+    "quickSetup": "Quick Setup",
+    "quickSetupDesc": "Create 6 alert rules for online Agent hosts: CPU(>80%)/Memory(>80%)/Disk(>90%)/TIME_WAIT(>15000)/CLOSE_WAIT(>200)/Process detection.",
+    "quickSetupConfirm": "Confirm Create",
+    "quickSetupCreating": "Creating…",
+    "quickSetupSuccess": "Default alert rules created successfully",
+    "quickSetupFailed": "Creation failed, check if Agent hosts are online",
+    "silenceDesc": "Quick silence from alert list",
+    "enabled": "Enabled",
+    "disabled": "Disabled",
+    "pageInfo": "Page {{page}}/{{totalPages}}, {{total}} total",
+    "addRule": "Add Rule",
+    "noRules": "No alert rules",
+    "noRulesHint": "Click 'Add Rule' or 'Quick Setup' to create",
+    "durationSeconds": "{{count}}s duration",
+    "contactsTitle": "Contacts",
+    "peopleCount": "{{count}} people",
+    "addContact": "Add Contact",
+    "addGroup": "Add Group",
+    "noContacts": "No contacts yet, click 'Add Contact' to create",
+    "noGroups": "No contact groups yet, click 'Add Group' to create",
+    "minutesShort": "{{count}}m",
+    "prevPage": "Previous",
+    "nextPage": "Next"
   }
 }