Feature/proper about window

[s0ph0s-dog]

This PR adds a proper macOS About window to this application, which:

• is a window;
• shows the app icon, version, copyright info, and release channel;
• closes when you press escape;
• and offers a real button to copy the version string to the pasteboard.

Right now, it is properly translated for English. I have no idea how the Telegram translation framework works, and I would greatly appreciate guidance on how to implement it so that people who read other languages can benefit from this change as well.

Is this PR acceptable? Have I matched the code style? I would love to see this implemented, so if there's anything you don't like about how I've done it, please let me know so I can improve it :3

[CLAassistant]

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[s0ph0s-dog]

@overtake I’d love to hear what you think about this PR. If it’s a flat-out rejection, that’s totally fine—please let me know so I can make any changes you’d like, or just move on.