Summary

Add ability to copy exported image directly to clipboard.

Motivation

Quick sharing without saving to disk - paste directly into Slack, docs, etc.

Implementation

• Add "Copy" button next to Export button
• Use NSPasteboard to copy PNG data
• Show toast "Copied to clipboard"

Files to Modify

• ContentView.swift - Clipboard logic
• ToolBarView.swift - Add Copy button

Acceptance Criteria

• Copy button visible next to Export
• Clicking copies image to clipboard
• Toast notification confirms copy
• Image can be pasted in other apps

Story Points

2 points