Increase PNG quality to 100 in workflow to avoid artifacts on transpa…

[dimitrieh]

Summary

This PR fixes white border artifacts that appear on transparent PNG images after automated compression.

Fixes #4278

Problem

The calibreapp/image-actions workflow was using lossy PNG compression (80% quality via sharp/libvips), which can introduce white border artifacts at transparency boundaries. This was observed in PR #4273 where mcp-in-flowfuse.png developed a white border at the top after automated compression.

Timeline

Date	Event
Feb 2023	Workflow introduced in PR #447 claiming "lossless compression"
Feb 2023	Reality: calibreapp/image-actions default pngQuality was always 80% (lossy)
Dec 2025	White border artifact observed in PR #4273

Solution

Alternative to #4281 just changing quality of png to 100

Test plan

• Create a test PR with a transparent PNG image
• Verify the workflow runs successfully
• Verify no white border artifacts are introduced
• Verify PNG file size is reduced

Checklist

• I have read the contribution guidelines
• I have considered the performance impact of these changes
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
• For blog PRs, an Art Request has been created (instructions)

[github-actions]

Images automagically compressed by Calibre's image-actions ✨

Compression reduced images by 62.5%, saving 271.4 KB.

Filename	Before	After	Improvement	Visual comparison
src/blog/2025/11/images/mcp-in-flowfuse.png	217.1 KB	81.4 KB	62.5%	View diff
src/blog/2025/12/images/mcp-in-flowfuse.png	217.1 KB	81.4 KB	62.5%	View diff

[dimitrieh]

@ppawlowski created this one instead of #4300 :)

[dimitrieh]

test plan success

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