Column Split View (Chat + Terminal) #1741

PRD: Column Split View (Chat + Terminal)

Problem

Currently T3 Code shows the terminal as a bottom drawer that overlays the chat. In canvas mode, this breaks the spatial model — the terminal should be part of the column layout.

Goals

• Mod+W toggles terminal stacked below chat in the focused column
• Niri-style column stacking: chat on top, terminal on bottom
• Resizable split with spring-animated resize
• Collapse/expand with smooth transitions

User Stories

• As a user, I want to see chat and terminal simultaneously in one column
• As a user, I want to resize the split by dragging
• As a user, I want to collapse the terminal back to full chat with a shortcut

Requirements

• Keybind: Mod+W (matches Niri's stack-in-column shortcut)
• Split ratio persisted per-thread in localStorage
• Spring-animated resize (using @react-spring/web)
• Terminal retains state across collapse/expand
• Default split: 60% chat / 40% terminal

Success Metrics

• Terminal visible alongside chat without leaving canvas view
• Resize feels smooth (spring physics, no jank)
• Terminal state preserved across split toggles

Scope

• In scope: Vertical split within CanvasColumn, resize handle, Mod+W keybind
• Out of scope: Horizontal split, multiple terminals per column, drag-to-detach

Priority

Later milestone (after canvas MVP)