feat: Panel styling options

[ikorich]

Summary

Customize panel appearance (corner radius, borders, shadows).

Motivation

Users want more visual customization options.

Options to Add

• Corner radius (0-20px)
• Border width and color
• Shadow intensity
• Background patterns/gradients

Acceptance Criteria

• Corner radius slider
• Border customization
• Shadow controls
• Settings persist

Story Points

3 points