[FEATURE] /splitscreen command and button for custom multi-tab layouts

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When working on multiple projects at the same time, there's no way to control how Claude tabs are arranged. You can't choose whether tabs split side by side or up and down, or mix both.

Proposed Solution

A /splitscreen command and a UI button that lets you design your tab layout. You should be able to choose side-by-side, up-and-down, or a mix of both — and control how wide or tall each pane is.

Alternative Solutions

Currently the only workaround is manually resizing separate windows. There's no built-in layout control inside Claude Code.

Priority

High - Significant impact on productivity

Feature Category

Interactive mode (TUI)

Use Case Example

1. I have 3 projects open
2. I type /splitscreen or click the button
3. I choose: two tabs side by side on top, one full-width on the bottom
4. All projects visible at once, arranged exactly how I want

Additional Context

_No response_