feat(proxy): add modify proxy API endpoint and functionality

Implement proxy modification feature including:
- New modifyFrpcProxy function in config.go
- New ModifyProxyHandler in frpcProxyAct.go
- New API endpoint in router.go
- Updated API documentation in docs/api.md

docs/api.md

@@ -1042,6 +1042,91 @@ remote_port = 6000
 
 ---
 
+## Modify Proxy
+
+**Endpoint:** `/frpcAct/proxyMgr/modify`  
+**Method:** POST  
+**Content-Type:** application/json  
+**Auth Required:** Yes (token)  
+**Permission Level:** Admin
+
+Modify an existing proxy configuration for an frpc instance. The proxy configuration will be updated in the instance's config file.
+
+**Request Headers:**
+```
+X-Token: your_token
+X-Timestamp: 1704067200000
+```
+
+**Request Body:**
+```json
+{
+  "instanceID": "1",
+  "proxyInfo": {
+    "name": "ssh_proxy",
+    "type": "tcp",
+    "localIP": "127.0.0.1",
+    "localPort": "22",
+    "remotePort": "6001"
+  }
+}
+```
+
+| Header | Type | Required | Description |
+|--------|------|----------|-------------|
+| X-Token | string | Yes | Authentication token |
+| X-Timestamp | int64 | Yes | Client timestamp in milliseconds |
+
+| Field | Type | Required | Description |
+|-------|------|----------|-------------|
+| instanceID | string | Yes | Instance ID (the ID of the frpc instance) |
+| proxyInfo.name | string | Yes | Proxy name (used to identify which proxy to modify) |
+| proxyInfo.type | string | Yes | Proxy type (e.g., tcp, udp, http, https) |
+| proxyInfo.localIP | string | Yes | Local IP address to forward to |
+| proxyInfo.localPort | string | Yes | Local port to forward from |
+| proxyInfo.remotePort | string | Yes | Remote port on frps to expose |
+
+**Response:**
+```json
+{
+  "success": true,
+  "message": "Proxy modified successfully",
+  "data": {
+    "instanceID": 1,
+    "configPath": "./configs/superfrpc_user_my_frpc.toml",
+    "proxyName": "ssh_proxy"
+  }
+}
+```
+
+| Field | Type | Description |
+|-------|------|-------------|
+| instanceID | int | Instance ID |
+| configPath | string | Path to the configuration file |
+| proxyName | string | Name of the modified proxy |
+
+**Config File Format:**
+
+The proxy configuration in the config file will be updated to the following format:
+
+```toml
+[[proxies]]
+name = ssh_proxy
+type = tcp
+local_ip = 127.0.0.1
+local_port = 22
+remote_port = 6001
+```
+
+> **Note:** 
+> - The proxy configuration is updated in the existing config file
+> - The instance must already exist before modifying a proxy
+> - This endpoint does not modify the database, only the config file
+> - The frpc service needs to be restarted for changes to take effect
+> - The proxy name is used to identify which proxy to modify; if the proxy is not found, an error will be returned
+
+---
+
 ## Delete Proxy
 
 **Endpoint:** `/frpcAct/proxyMgr/delete`