[BUG] Incorrect diff colouring positioning when snacks_picker_list open

## Bug Description

Seems like `snacks_list_picker` is not being filtered out of the main window checks when a file is opened to display a diff.

## To Reproduce

Steps to reproduce the behavior:

1. Open nvim
2. Toggle snacks file explorer
3. Initiate a diff with `claudecode.nvim` on an unopened file
4. See incorrect colouring placement

## Expected Behavior

Diff colouring should be applied to the buffer containing the previous file state

## Environment

- Neovim version: 0.11.6
- Claude Code CLI version: 2.1.50
- OS: macOS 26.2
- Plugin version: 0.3.0

## Error Messages

N/A

## Additional Context