docs: digests on the site and in the README for 3.1

[pasichDev]

What

The site and the README catch up with 3.1:

• Site (docs/index.html):
  • A 3.1 banner replaces the 2.x-migration one; the migration link stays.
  • A Digests nav link.
  • A new "A digest of your work, on the home page" section with three screenshot tabs — Home, By area and By person.
  • Three cards: hand-off by number, seen marks, configuration.
  • The SessionStart card mentions the digest line.
  • The install guide says the home page is the digest and the list is at /tasks.
  • The test count goes from 510 to 555, with links to the 3.1 changelog and to docs/digests.md.
• README: the Digests section gets a home-page screenshot, a short list of what a digest does, and the By area / By person screenshots.
• Screenshot tabs are now scoped to their own section, since the page has two sets.

Why

3.1.0 is published, but the site and README still describe 3.0 and show no digest.

Testing

• Previewed the site locally: banner, nav, the new section and its tabs. Each set of tabs switches independently.
• The screenshots are of invented demo data from the Docker image built from main.
• npm test: 555 passing, including the example and private-word guards.