[Desktop] Allow resizing the divider between split sessions

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

Summary

In the redesigned desktop app, two sessions can be shown side-by-side, but the divider between them appears to be fixed at 50/50 and cannot be dragged. It would be great to make this divider resizable, the same way panes inside a single session are.

Motivation / Use case

The two sessions usually play different roles — for example:

• Left: monitoring logs / watching a long-running task
• Right: active coding and chat

A fixed 50/50 split wastes space on the "passive" side and cramps the "active" side. Being able to drag the divider (e.g. 70/30) would make parallel-session workflows much more practical.

Current behavior

• Hovering the divider between two sessions does not change the cursor.
• Dragging has no effect; the split stays at 50/50.
• Panes within a single session (terminal / chat / editor) resize fine — only the session-level divider is fixed.

Proposed Solution

Expected behavior

• Hovering the session divider shows a resize cursor.
• Dragging adjusts the ratio freely (with a sensible minimum width).
• Ratio is remembered per window / across restarts.

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

Performance and speed

Use Case Example

_No response_

Additional Context

Environment

• Claude Code Desktop (Windows 11)
• Version: Claude 1.3109.0 (35cbf6) 2026-04-16T20:32:01.000Z

Additional context

Related to the April 2026 desktop redesign that made in-session panes draggable. This request extends the same behavior to the session-level split.