refactor(frpc): restructure instance management and config handling

- Move instance-related structs and functions to config.go
- Remove serverAddr, serverPort, and authMethod from database schema
- Implement new config parsing and encoding with nested key support
- Update service management to use instanceID instead of username/name
- Add GetServiceNameByInstanceID helper function
- Update API documentation for auth.method field change

docs/api.md

@@ -552,8 +552,8 @@ Modify fields in the `[common]` section of the frpc configuration file. Only `[c
   "modifiedData": {
     "server_addr": "192.168.1.1",
     "server_port": "7000",
-    "auth_method": "token",
-    "auth_token": "my_secret_token"
+    "auth.method": "token",
+    "auth.token": "my_secret_token"
   }
 }
 ```
@@ -1149,7 +1149,7 @@ X-Timestamp: 1704067200000
       "name": "my_frpc",
       "serverAddr": "127.0.0.1",
       "serverPort": "7000",
-      "auth_method": "token",
+      "auth.method": "token",
       "bootAtStart": true,
       "runUser": "root",
       "configPath": "./configs/superfrpc_user_my_frpc.toml",
@@ -1185,14 +1185,14 @@ X-Timestamp: 1704067200000
 | name | string | Instance name |
 | serverAddr | string | frps server address (admin/superuser only) |
 | serverPort | string | frps server port (admin/superuser only) |
-| auth_method | string | Authentication method (admin/superuser only) |
+| auth.method | string | Authentication method (admin/superuser only) |
 | bootAtStart | bool | Auto-start on system boot |
 | runUser | string | User to run the frpc instance as |
 | configPath | string | Path to the configuration file |
 | createdAt | string | Instance creation time (ISO 8601 format) |
 | createdBy | string | Username of the user who created this instance |
 
-> Note: Visitor users do not see sensitive information (serverAddr, serverPort, auth_method).
+> Note: Visitor users do not see sensitive information (serverAddr, serverPort, auth.method).
 
 ---