Get rid of iOS save mixer channel

[ann0see]

Describe the bug

The save mixer channel feature on iOS just opens a file explorer window (QT inbuilt) in the iOS sandbox. This seems useless for me.

To Reproduce

On iOS save mixer channels.
See that a non native dialog pops up.

Expected behavior

A share sheet or similar should show up. Since for now I don't think it's worth implementing the share sheet, we should ifdef this feature.

Screenshots

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Operating system

iOS 26

Version of Jamulus

3.11.0 from App Store

Additional context