feat(tui): add double-click to copy word

[ndraiman]

Summary

Double-click on a word to copy it to clipboard, using the same selection mechanism as drag-to-copy.

Changes

• Detect double-clicks by tracking click time/position (400ms threshold)
• Find word boundaries by scanning the render buffer for word characters
• Create selection using existing startSelection/updateSelection APIs
• Existing onMouseUp handler automatically copies to clipboard with toast notification

Behavior

• Works anywhere drag-to-copy selection works
• Word characters: letters, digits, underscore, hyphen (unicode-aware)
• Respects OPENCODE_EXPERIMENTAL_DISABLE_COPY_ON_SELECT flag

https://github.com/user-attachments/assets/945ce58d-ec9d-4135-b556-896fccfe01b8