Automated UI testing

[Uncommon]

Start on a suite of UI tests to complement the existing unit tests.

• [x] Add accessibility IDs everywhere
• [ ] Open a window on a simple repository and verify all UI elements
  • [x] Sidebar
  • [x] Title bar
  • [ ] History list
  • [x] File list
  • [x] Commit summary
  • [ ] File previews