Add copy mode to vf-tui

[snimu]

Description

Press c to enter copy mode. The completion column will be focussed by default. There, you can:

• Press Tab to switch between the completion and prompt column
• Highlight text with the mouse or Shift+(left/right arrow)
• Press c again to copy text (if any text is highlighted, copies that highlighted text, otherwise copies the whole column text)
• Esc to leave the copy mode
• q to quit vf-tui

Images:

Completion column selected:

Prompt column selected:

Copied highlighted text:

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test improvement

Testing

• All existing tests pass when running uv run pytest locally.
• New tests have been added to cover the changes

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

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Note

Introduces interactive Copy Mode in the TUI for extracting prompt/completion text.

• New CopyScreen modal with read-only TextArea columns; supports selection via mouse/Shift+Arrows and copying via c (Tab switches columns, Esc closes, q quits)
• ViewRunScreen: binds c to open Copy Mode, tracks plain prompt/completion text for copying
• Adds clipboard and selection helpers; expands CSS and imports for copy UI
• Docs (overview.md, faqs.md) updated to mention Copy Mode; small formatting fix

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