Add Ko-fi link to FOSS About screen

[Pygmalion69]

Summary

• add a distribution-specific SHOW_DONATE_LINK BuildConfig flag
• show a clickable Donate via Ko-fi link in the About dialog for FOSS builds
• keep the Play About dialog unchanged

Verification

• FOSS variants compile the conditional link with the flag enabled
• Play variants compile the same shared dialog with the flag disabled
• URL: https://ko-fi.com/U7U372JJ