Cap transcript width at 120 columns for readability

[n-WN]

Summary

• Adds a MAX_TRANSCRIPT_WIDTH (120 columns) cap to the TUI transcript rendering area
• On wide terminals, text now wraps at 120 columns instead of stretching across the full terminal width
• Applied consistently across rendering, viewport storage, line count, and height calculations

Test plan

• cargo check passes
• Existing tests pass (130/130, 2 pre-existing failures unrelated)
• Visually verify on terminals wider than 120 columns that text wraps earlier
• Verify no change on terminals narrower than 120 columns