feat: add ability to copy input to clipboard

[zach-fuller]

Summary

This PR introduces a keyboard shortcut for copying the current input. This feature allows users to easily copy composed messages (including text and attachment references).

Changes

Core Implementation:

• Adds InputCopyCommand with ctrl+shift+c default keybind
• Implements Copy() method in editor component with toast feedback
• Adds command execution handler

Documentation:

• Updates keybind documentation to include new input_copy shortcut