feat(ui): switchable foreground/background colors

[csehatt741]

Summary

A new feature was implemented to switch foreground/background colors

• a new hotkey, toggleFillColor, was added to toggle fill color by x
• new fields, activeColor, color1 and color2, were added to canvasSettingsSlice to manage fill color
• ToolFillColorPicker component was refactored to handle foreground/background colors
• new tanslations, hotkeys.canvas.setFillToWhite, fillColor1 and fillColor2, were added to en.json

Related Issues / Discussions

Closes #8381

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

[psychedelicious]

We still want the d hotkey, but its behaviour should change. The conventional action for d is to set background to black and foreground to white. Then x toggles between them.

Screen.Recording.2025-09-01.at.2.34.54.pm.mov

Looks great besides that small change

[psychedelicious]

Awesome, TY!