feat(iplimit): gate IP limit on fail2ban and reset stale limits

Per-client IP limit only enforces where fail2ban is installed, so the panel now reports enforceability and disables the field otherwise:

- Add GET /panel/api/server/fail2banStatus (enabled/installed/usable/windows), cached 30s.
- ClientFormModal and ClientBulkAddModal disable the IP Limit input when not usable and show a hover tooltip; Windows gets a platform-specific message instead of the bash-menu hint.
- One-time migration ResetIpLimitNoFail2ban zeroes existing client limitIp (inbound settings JSON + clients table) on hosts without fail2ban, where the limit never applied.
- Drop the recurring '[LimitIP] Fail2Ban is not installed' warning.
- Add limitIpFail2banMissing/limitIpFail2banWindows/limitIpDisabled across all 13 locales.

frontend/public/openapi.json

@@ -3316,6 +3316,45 @@
         }
       }
     },
+    "/panel/api/server/fail2banStatus": {
+      "get": {
+        "tags": [
+          "Server"
+        ],
+        "summary": "Reports whether per-client IP limits can be enforced on this host. The panel uses it to gate the \"IP Limit\" field, since enforcement depends on Fail2ban being installed.",
+        "operationId": "get_panel_api_server_fail2banStatus",
+        "responses": {
+          "200": {
+            "description": "Successful response",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "type": "object",
+                  "properties": {
+                    "success": {
+                      "type": "boolean"
+                    },
+                    "msg": {
+                      "type": "string"
+                    },
+                    "obj": {}
+                  }
+                },
+                "example": {
+                  "success": true,
+                  "obj": {
+                    "enabled": true,
+                    "installed": true,
+                    "usable": true,
+                    "windows": false
+                  }
+                }
+              }
+            }
+          }
+        }
+      }
+    },
     "/panel/api/server/cpuHistory/{bucket}": {
       "get": {
         "tags": [