Fix: Issue #2709 Change default value of stealth_pnginfo_option to "None" to prevent unintended alpha channel watermarking

[CircleD5]

This pull request addresses #2709 regarding the unintentional embedding of stealth metadata (a hidden watermark) into the alpha channel of generated images.

Reason:

• The default setting of "Alpha" caused stealth metadata to be embedded in the alpha channel even when users were unaware of it.
• This has led to real-world consequences, including targeted harassment of artists who assumed they had removed all metadata.
• Setting the default to "None" ensures that users must explicitly opt in if they want stealth metadata.
• This aligns with privacy expectations and avoids unintentional leaks of prompt or generation details.

Impact:

• This change does not remove the feature, but makes it opt-in instead of opt-out.
• No effect for users who have already configured this setting manually.