PAINTROID-802: Add Clear button to Clipboard tool

[agsyna]

This PR adds a Clear action to the existing Clipboard Tool, allowing users to explicitly clear copied or cut content without affecting the canvas.

PAINTROID 802

New Features and Enhancements

• Clear Clipboard Action: Added a Clear button to the Clipboard Tool options
• Clipboard state is reset safely without modifying the canvas

Testing

• Added integration tests covering
  • Clear After Copy
  • Clear After Cut
  • Paste After Clear
  • Multiple Clear operations on empty clipboard
• Verified canvas integrity using undo stack stability

Checklist

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