Allow configuring the shell used by the Run button in Claude Code desktop app

### Preflight Checklist

- [x] I have searched [existing requests](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20label%3Aenhancement) and this feature hasn't been requested yet
- [x] This is a single feature request (not multiple features)

### Problem Statement

On Windows, the Run button on code blocks always uses PowerShell. There is no way to change it to Git Bash or another shell.

### Proposed Solution

Please add a setting (e.g. in settings.json or the app UI) to configure which shell the Run button uses.

### Alternative Solutions

_No response_

### Priority

Medium - Would be very helpful

### Feature Category

Configuration and settings

### Use Case Example

1. I work on Windows with Git Bash as my main shell.
2. Claude generates bash commands using `export`, `&&`, and heredocs that only work in bash — not PowerShell.
3. When I click Run, it opens PowerShell and the command fails.
4. I have to manually open Git Bash and paste the command there.
5. If the Run button used Git Bash, I could click once and the command would just work.

### Additional Context

_No response_

No implementation files or tests are named. Start with the desktop app's Run button flow and the settings.json or app UI configuration path, then trace how Windows selects PowerShell. Done means users can choose Git Bash or another shell and the Run button executes commands through that selection.

Written by the indexing model from the issue text.