feat: add color blend unmixer tool

[nsbgg]

Summary

This PR adds a Color Blend Unmixer tool.

The tool calculates possible semi-transparent foreground colors from a given background color and blended/resulting color. It can help reverse-engineer which RGBA color could have produced a visible blended color.

This is based on the idea from nsbgg/blend-unmixer, adapted as a tool for devtool-plus.

Changes

• Added Color Blend Unmixer tool
• Added UI for entering background and blended/resulting colors
• Added logic to calculate possible foreground colors and opacity values
• Outputs possible RGBA/hex results
• Integrated the tool into the existing devtool-plus tool list

Testing

• Tested manually with known background/blended color combinations
• Verified that generated RGBA results reproduce the expected blended color
• Ran project checks locally

Notes

There was no existing feature request for this, but I thought it would be a useful addition for color/design workflows.