feat(ui): add live output viewer

[unrelentingfox]

Summary

• replace the editable task output view with a read-only full-terminal overlay
• add Vim-like vertical and horizontal navigation with pinned help and borders
• tail the full log live, pause following while scrolled up, and resume at the bottom
• bound retained viewer output and clean up follower resources on close

Testing

• pnpm run check
• 212 tests passed
• manually verified full history, live following, navigation, and pinned layout