feat(tui): collapse large composer pastes

[BrendanChou]

Summary

• automatically collapse pastes of at least 1,000 characters or more than 8 lines into compact blue tokens with character and line counts
• preserve the exact original text structurally for submission, copy, history, image offsets, saved drafts, and the external editor
• make tokens atomic for cursor movement and deletion, and expand them on click
• use semantic pasted text for @ and $ mention boundaries
• add composer.paste_display = "auto" | "inline" with startup and live-reload propagation

Validation

• just test -p tact (801 passed)
• just clippy -p tact
• just check-fmt
• independent correctness and simplicity reviews: no actionable findings

[clabby]

I'm not a huge fan of this behavior personally. The composer does cap the number of lines it'll render at once, were you running into issues?