Fix "Flash Window on Message" notification feature

[google-labs-jules]

This PR fixes the "Flash Window on Message" feature which was previously disabled and marked as a TODO.

Changes:

• In source/index.ts, the notification IPC handler now checks the flashWindowOnMessage config and triggers mainWindow.flashFrame(true) if enabled and not on macOS.
• In source/menu.ts, the menu item visibility is updated to !is.macos (previously is.development) and the TODO comments are removed.

This ensures that users on Windows and Linux receive a taskbar flash when a new message notification arrives, if the option is enabled.

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PR created automatically by Jules for task 5825506827835256528 started by @raztronaut

[google-labs-jules]

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