Add single-instance tauri plugin

[aceArt-GmbH]

Description

Uses official tauri plugin.

Currently, if minimized and hidden, it will show but not unminimiz.
Also after being shown by a second instance, the tray menu will continue to say "Show Cinny" while it should "Hide Cinny" as it is now shown.
cc tauri-apps/plugins-workspace#280

Fixes #210

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[kfiven]

Hi @aceArt-GmbH , Could you please rebase this on top of latest changes in dev?

[aceArt-GmbH]

@kfiven rebased it onto main