feat(print): Column visibility toggle for print output

[hangtime79]

Summary

Allow hiding columns specifically for print without modifying main view.

Use Cases

• Hide "Internal Cost" column for client handouts
• Hide "Assignee" to save horizontal space
• Show only essential columns for summary view

UI Design

• Checkbox list of available columns in Print Studio sidebar
• "Client-safe" preset (hide internal columns)
• Changes apply only to print, not main view

Key Insight

This is the single biggest friction point for users who want to print "client-safe" versions of their internal charts.

References

Smartsheet's column picker is best-in-class for this.

Parent Epic

Part of #86