Cross-section canvas UI polish

[deannagelosi]

• Reset button is now "Reset Picks" and clears only the weft pick paths, keeping warp structure and system assignments intact
• Warp index labels: numbered labels (1, 2, 3...) appear below each warp column so users can identify warp positions at a glance
• Bezier gradient contrast: toned down the darker-to-lighter color shift along weft curves so the gradient is subtler and less distracting