Admin UI: Use headings for easier screen reader navigation

Screen reader users can navigate quickly through a page by going through an outline of headings. We should update the dashboard & settings pages to use a semantic heading structure. [See the wp core guidelines.](https://make.wordpress.org/core/handbook/best-practices/coding-standards/accessibility-coding-standards/#heading-structure)

Given the guidelines above, we should

- Set a single h1 page title
- Use correct headings for card titles (and only the titles, buttons should not be in headings)
- Remove headings from stats numbers
- Use headings for settings cards

These can be hidden visually & only shown for screen-readers if we need to.

Start with the dashboard and settings pages and review the linked WordPress core heading-structure guidelines. Update the page structure to use one h1, appropriate card-title headings without buttons, no headings on stats numbers, and headings for settings cards; headings may be visually hidden when needed for screen readers.

Written by the indexing model from the issue text.