feat(fwd-proxy): Squid cache stats card — GET /forward-proxy/stats + squidclient mgr:counters

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

management-ui/src/i18n/locales/en/common.json

@@ -975,6 +975,18 @@
     "deleteConfirm": "Really delete ACL {{name}}?",
     "emptyTitle": "No forward-proxy ACLs yet.",
     "emptyDesc": "Default with no ACLs: only localnet (10/8, 172.16/12, 192.168/16) is allowed out. Add an ACL to selectively allow or block specific domains/IPs/ports.",
+    "statsCard": {
+      "title": "Squid cache statistics",
+      "clientRequests": "Client requests",
+      "cacheHits": "Cache hits",
+      "cacheHitPct": "Hit rate",
+      "clientErrors": "Client errors",
+      "bytesIn": "Bytes received",
+      "bytesOut": "Bytes sent",
+      "serverRequests": "Backend requests",
+      "serverErrors": "Backend errors",
+      "sinceRestart": "Since last restart"
+    },
     "actions": {
       "allow": "allow — permit access",
       "deny":  "deny  — block access"