feat: TUI overhaul — frameless layout, left-accent tools, bottom-pinned status

- Remove outer ╭╰ border frame, replace with thin ─── separators
  colored by state (blue=streaming, orange=permission, purple=planning)
- Tool blocks use left accent bar instead of full rounded border
- Activity spinner + status always pinned above bottom separator
- Task checklist pinned near bottom with strikethrough on completion
- Arrow-key permission picker (←→ select, enter confirm, y/n/a shortcuts)
- Scroll indicator embedded in bottom separator (─── ↑ 42% ───)
- Stable height math: every dynamic element counted in viewport budget
- Wider content area (full terminal width, no frame padding)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: halfaipg