chore: add data attributes to UI components for testing
Add data-* attributes to various components for easier testing and debugging:
- ChangedFilesList: data-focused, data-checked, data-file-path
- DiffPanel: data-selected, data-file-path
- RevisionGraph: data-focused, data-selected, data-checked, data-expanded,
data-change-id on revision cards; data-focused, data-stack-id on stacks