test: add accesssibility checks and fix the violations they found

[nva138]

Part of #82, the accessibility bit.
Adds the axe check on the main editor state, plus two keyboard tests: can you work the toolbar without a mouse, and is the focus actually visible.

axe turned up five real things, so I just fixed them here, a red check can't merge anyway. The resize handles had no accessible name, the page overlay had an aria-label sitting on a div with no role so screen readers just ignore it, and the demo page was missing a main and an h1 and skipped a heading level.

[vitormattos]

The accessibility tests added here are useful, but they are not running in CI yet.

Since #82 also includes adding Playwright to GitHub Actions, I think it is better to keep this PR open and create a separate PR first for the Playwright CI workflow.

This will also make the code review easier. One PR can focus only on the Playwright setup, and this PR can focus on the accessibility tests and fixes.

After the CI PR is merged, we can update this PR so these tests run as part of the checks.

[nva138]

I'll do the CI workflow as a separate PR first and update this one afterwards

[nva138]

Updated with main, so the Playwright job runs here too.
Also set workers: 1 the suite started failing randomly locally, a different test each run. With one worker it's stable.