notesnook: add Linux support using AppImage

[ta7mid]

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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.
• brew style --fix <cask> reports no offenses.

Additionally, if adding a new cask:

• Named the cask according to the token reference.
• Checked the cask was not already refused (add your cask's name to the end of the search field).
• brew audit --cask --new <cask> worked successfully.
• HOMEBREW_NO_INSTALL_FROM_API=1 brew install --cask <cask> worked successfully.
• brew uninstall --cask <cask> worked successfully.

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output, including zap stanza paths; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

Claude Code with Claude Fable 5.1 was used to inspect the app's source code to figure out the zap stanza paths for the AppImage and to find if the AppImage auto-updates. The zap paths were verified locally by running the app and checking their existence with find ~ -maxdepth 4 -iname '*notesnook*'.

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