[Bug]: crossplay config variable has no effect. -crossplay hardcoded in startparameters with no way to disable via config

### Severity

severity: low

### Reproducibility

reproducible: always

### Regression

regression: no

### Affects latest release

latest-release: yes

### User story

As a server owner running a Steam-only Valheim server, I want the crossplay config variable to actually control whether -crossplay is passed to the server binary, so that I can disable Playfab relay networking and allow players to connect via direct IP without having to manually override startparameters.

### Script name

vhserver

### Game

Valheim

### Linux distro

Other, Ubuntu 26.04

### Command

command: start

### Expected behavior

-crossplay should not appear in the launch command when crossplay="false" is set.

### Actual behavior

-crossplay is always present regardless of the crossplay config value. The server initialises Playfab/Xbox Live networking, routes all connections through Microsoft's relay network, and becomes unreachable via direct IP for Steam-only players.

Impact:
Steam-only dedicated servers are completely unreachable by IP. Clients see the server as permanently offline in the favourites browser. No connection attempts reach the server process at all.

### Further information

Workaround:

Override startparameters in the instance config with -crossplay removed:
`startparameters="-name '${servername}' -password ${serverpassword} -port ${port} -world ${worldname} -public ${public} -savedir '${savedir}' -saveinterval ${saveinterval} -backups ${backups} -backupshort ${backupshort} -backuplong ${backuplong} -instanceid ${instanceid} ${logFile:+ -logFile '${logFile}'} ${worldmodifiers:+ ${worldmodifiers}}"`

Suggested fix:
Replace the hardcoded -crossplay in _default.cfg with a conditional so the existing crossplay config variable actually controls it:
`${crossplay:+$([ "$crossplay" = "true" ] && echo "-crossplay")}`

Or more simply, remove -crossplay from the default startparameters entirely and document that users who need crossplay should add it manually, making crossplay opt-in rather than opt-out, which is the safer default for Steam-only servers.

### Pre-checks

- [x] I ran update and validate before reporting this issue.

### Relevant log output

```shell

```

### Steps to reproduce

Steps to reproduce:

Install a fresh vhserver
Set crossplay="false" in lgsm/config-lgsm/vhserver/vhserver.cfg
Start the server with ./vhserver start
Run ./vhserver details and inspect the Command-line Parameters section