Fix diff viewer related changes

[FourWindff]

Summary

Disable text selection on UI chrome elements in the Diff Viewer to prevent accidental selection of non-content text (headers, filenames, file list items) and improve the
overall interaction experience.

Changes

• ChangedFilesList.tsx — add user-select: none to the file list container
• DiffViewerDialog.tsx — add user-select: none to the dialog header title and "Changed Files" label
• ScrollingDiffView.tsx — add user-select: none to the file section header

Test Plan

• Open the Diff Viewer and attempt to select header titles, filenames, and file list items — verify they cannot be selected
• Confirm that diff content areas remain selectable as before

[johannesjo]

Thank you very much!