feat(userMgr): add user removal functionality

- Implement RemoveUser function in database.go to delete users
- Add RemoveUserHandler to handle HTTP requests for user removal
- Update API documentation with new /userMgr/remove endpoint
- Add route for user removal in router.go

docs/api.md

@@ -173,7 +173,7 @@ X-Timestamp: 1704067200000
 ```json
 {
   "success": true,
-  "message": "user created successfully",
+  "message": "User created successfully",
   "data": {
     "userID": 2,
     "username": "new_user",
@@ -184,6 +184,37 @@ X-Timestamp: 1704067200000
 
 ---
 
+## Remove User
+
+**Endpoint**: `/userMgr/remove`  
+**Method**: POST  
+**Content-Type**: application/json  
+**Auth Required**: Yes (token)  
+**Permission Level**: Superuser
+
+**Request Headers**:
+```
+X-Token: your_token
+X-Timestamp: 1704067200000
+```
+
+**Request Body**:
+```json
+{
+  "targetUserID": 2
+}
+```
+
+**Response**:
+```json
+{
+  "success": true,
+  "message": "User removed successfully"
+}
+```
+
+---
+
 ## Create frpc Instance
 
 **Endpoint:** `/frpcAct/instanceMgr/create`