zettlr: add app_image config

[SMillerDev]

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After making any changes to a cask, existing or new, verify:

• The submission is for a stable version or documented exception.
• brew audit --cask --online <cask> is error-free.
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• Named the cask according to the token reference.
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• AI was used to generate or assist with generating this PR. Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes, including zap stanza paths.

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Replacement for #264537 to get some AppImages going here.

[SMillerDev]

Needs Homebrew/brew#22312