[FEATURE] Per-turn diff view in Claude Code Desktop (parity with CLI /diff)

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

In the Claude Desktop app (Code tab), the diff view only offers "All changes vs master" and "Uncommitted". Neither tells me what a specific prompt changed:

• "All changes vs master" includes everything on the branch, including work from earlier prompts that I already reviewed.
• "Uncommitted" only works as a per-prompt view if I commit or stash manually before every prompt, which is easy to forget.

The CLI already solves this: /diff has per-turn tabs (T1, T2, ...) that show what Claude edited in each turn. Those tabs are reconstructed from the session's tool calls, so they don't depend on git state. Desktop has no equivalent, and a CLI session started next to a Desktop session can't show the Desktop session's turns.

Proposed Solution

Add a per-turn / "This session" option to the Desktop diff view:

• A selector entry per turn (T1, T2, ...), like in CLI /diff, ideally showing the prompt text next to each turn.
• Same behavior as the CLI: independent of git state, so it still works after manual edits or git add.

Alternative Solutions

I could work in the CLI to get this. But I strongly prefer Claude Desktop.

Priority

High - Significant impact on productivity

Feature Category

Other

Use Case Example

I run a prompt in Claude Desktop -> Code and want to review exactly what that run changed before I continue. Because the diff view only compares against the whole branch or the uncommitted state, I can't isolate one prompt's changes without extra manual git steps.

Additional Context

• Related: #52179 (diff base selector)