feat: thread terminal pane parity

[BearHuddleston]

What changed

This adds a thread terminal pane to the TUI and brings it much closer to Desktop behavior where that maps cleanly to the terminal surface.

Key changes:

• add a real xterm-backed TUI terminal surface instead of a plain transcript view
• support thread-scoped terminal state, multiple tabs, split panes, and full-screen terminal mode
• align terminal scrolling, selection, and copy behavior more closely with the session pane and composer UX
• share terminal state/activity helpers in client-core so the TUI and web app use the same terminal state model
• tighten terminal controls and toolbar icons in the TUI header

Why

The existing TUI did not have Desktop-like terminal support for thread work. This change closes that gap and fixes a long chain of interaction issues around focus, selection, split behavior, scrolling, and PTY rendering.

Impact

• TUI users can open, split, scroll, resize, focus, and full-screen thread terminals directly in the main thread view
• terminal selection/copy behavior is much closer to other selectable surfaces in the app
• shared terminal state logic reduces divergence between TUI and web terminal state handling

Root cause

The earlier TUI approach rendered PTY output as normalized transcript text, which could not support correct cursor placement, selection stability, split-pane interaction, or session-pane-like scrolling behavior.

Validation

• bun fmt
• bun lint
• bun typecheck

Demo

2026-04-08.12-30-17-github.mp4