feat(frpc): improve boot service error handling and response

- Add bootServiceError field to system config modify response when boot service operations fail
- Include instanceID in list instances response
- Update error message for unknown modify type
- Document bootServiceError behavior in README

README.md

@@ -355,6 +355,33 @@ Modify system-level settings such as instance name, boot-at-start configuration,
 }
 ```
 
+**Response with bootServiceError (when boot service operations fail):**
+```json
+{
+  "success": true,
+  "message": "System config modified successfully",
+  "data": {
+    "instanceName": "new_frpc_name",
+    "instanceID": 1,
+    "configPath": "./configs/superfrpc_user_new_frpc_name.toml",
+    "bootAtStart": true,
+    "runUser": "www-data",
+    "bootServiceError": "Failed to create boot service: unsupported init system"
+  }
+}
+```
+
+> **Note about bootServiceError:**
+> - The `bootServiceError` field appears in the response when boot service operations (create/remove) fail
+> - This field is only present when there is an error; it will not be included in the response if operations succeed
+> - The overall request still returns `success: true` because the system config modification (database update, config file rename, etc.) succeeded
+> - Common scenarios where `bootServiceError` may appear:
+>   - Enabling `bootAtStart` when the init system is not supported (e.g., on Windows or systems without systemd/init.d)
+>   - Disabling `bootAtStart` when the service file cannot be removed due to permission issues
+>   - Changing instance name or run user when `bootAtStart` is enabled (requires removing old service and creating new one)
+> - The error message provides detailed information about what went wrong, which should be displayed to the user
+> - Frontend applications should check for this field and display the error message to the user, even though the request was technically successful
+
 ---
 
 ### 6. List frpc Instances