missing resolv.conf symlink breaks DNS for applications that integrate with systemd-resolved

### Description

I use Mullvad VPN and DNS fails on boot to where it says I have no internet. When I disconnect and reconnect this fixes the issue.

Checking `resolvectl status` shows that it's using `resolv.conf mode: foreign`. Mullvad checks whether `/etc/resolv.conf` is a symlink to a known systemd-resolved stub path before using the D-Bus API. When it's a regular file, the check fails and Mullvad falls back to directly writing `/etc/resolv.conf`, which races with systemd-resolved on boot and causes DNS failures.

```
Can't manage DNS using systemd-resolved: systemd-resolved operation failed
Managing DNS via /etc/resolv.conf
```

Issue #79 enabled `dns=systemd-resolved` in NetworkManager and started `systemd-resolved.service`, but didn't create the resolv.conf symlink. `/etc/resolv.conf` is still the regular file shipped by the `filesystem` package.

Creating the symlink manually fixed it for me:

```
sudo ln -sf /run/systemd/resolve/stub-resolv.conf /etc/resolv.conf
```

After a reboot, `resolvectl status` reports `resolv.conf mode: stub` and Mullvad manages DNS through systemd-resolved without issues:

```
Managing DNS via systemd-resolved
```

Looking at the Arch Wiki, it says this symlink is ["the recommended mode of operation"](https://wiki.archlinux.org/title/Systemd-resolved#DNS) when using systemd-resolved, so it seems like this is the right solution.

### System info

- CachyOS, kernel 6.19.10-1-cachyos
- NetworkManager 1.56