Desktop: Fix errors found by automated accessibility testing

[personalizedrefrigerator]

Summary

This pull request:

1. Sets up automated accessibility scanning with @axe-core/playwright (docs).
   • For now, this scans the "General" and "Plugins" tabs of the settings screen and the main screen with an open note.
2. Fixes WCAG errors found by the scan.

Note

This pull request includes code from #11218.

Change summary

• Missing lang attribute on HTML.
  • Fixed by merging Desktop: Accessibility: Declare app locale with the HTML lang attribute #11218 into this branch.
• Avoid empty aria-labels:
  • Fixed by using aria-hidden={true} instead.
• Missing role='main' and role='navigation' elements.
  • Fixed by adding role='navigation' to the sidebars and role='main' to the note editor.
  • The two role='navigation' elements are distinguishable by their different aria-labels.
• aria-details={id} sometimes points to an ID that isn't in the document.
  • Fixed by ensuring that the details element always exists. Previously, it was only created if it had content.
• aria-setsize on unsupported element.
  • Removed aria-setsize from the note list and sidebar content containers. (aria-setsize is already correctly present on children of the sidebar and note list).
• Missing label for note viewer.
  • Added aria-label to the note viewer.
• Missing label for the Markdown editor's content.
  • Added aria-label to the Markdown editor's content.
• Contrast issues:
  • Made colorFaded and color3 darker in the light theme.
  • Previously, the following elements had contrast issues in light mode:
    • Setting labels (3.18:1, expected 4.5:1)
      • Before:
      • After:
    • Plugin version text (3.47:1, expected 4.5:1)
      • Before:
      • After:
    • Toolbar text labels (e.g. the language code for the spellchecker toolbar item), (3.79:1, expected 4.5:1)
      • Before:
      • After:

Additionally, while running tests in dark mode (currently disabled because of contrast issues not included below):

• Improved contrast of the "Plugins" settings sidebar divider.

[personalizedrefrigerator]

CI errrors in this CI run should be fixed by #11224.

[laurent22]

Conflict on styles/index.scss

[personalizedrefrigerator]

The Android APK build failure should be fixed by #11294.

[laurent22]

Sorry, more conflicts now. Other than that it's ready to merge