Proxy settings get reverted

### What Happened?

My target proxy setting is the following (given as a Dconf dump):
```ini
[system/proxy]
mode='manual'

[system/proxy/socks]
host='localhost'
port=9050
```
I can enter this configuration in switchboard and if I then check the Dconf content, it will have these settings stored. But whenever I re-enter the network module of switchboard, the settings get reverted to:
```ini
[system/proxy]
mode='auto'

[system/proxy/socks]
host='localhost'
port=9050
```
The `manual` setting for `mode` does not stick. Switchboard keeps changing this back to `auto`.

### Steps to Reproduce

1. open switchboard, navigate to the network module, navigate to proxy settings
2. choose manual proxy mode, enter details for SOCKS: localhost, port 9050
3. quit switchboard
4. re-enter switchboard, navigate again to the proxy settings

### Expected Behavior

The proxy settings should keep the configuration I entered earlier.

### OS Version

8.x (Circe)

### OS Architecture

arm64 (on Raspberry Pi, Apple Silicon, etc.)

### Session Type

Secure Session (Wayland)

### Software Version

Latest release (I have run all updates)