build(windows): during installation remove autostart entry before updating/reinstalling, to prevent duplicates

[ErikBjare]

No idea if this fixes it.

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:rocket: This description was created by Ellipsis for commit a00b53a83a615b83c684b66f422d4991605f236b

Summary:

This PR updates the activitywatch-setup.iss script to remove existing autostart entries during installation, preventing duplicates.

Key points:

• Updated activitywatch-setup.iss to remove existing autostart entry during installation.
• Prevents duplicate autostart entries on reinstall/update.

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