[ZulkerNyne]

Summary

• Enable custom color configuration for pie and doughnut chart series
• Remove pie/doughnut exclusion from the color picker UI in the Visuals tab
• Use coloredSeries config in chart-data-processing.ts for pie/doughnut slice colors, falling back to the default palette

Closes #689

Test plan

• Create a pie chart and verify the Colors section appears in the Visuals tab
• Change a slice color and verify it updates in the chart preview
• Create a doughnut chart and verify the same behavior
• Verify bar/line/area charts still work as before
• Verify table charts still hide the color picker