Redesign the README: header, hero image, scannable features

[Lucifix]

This gives the README the layout popular self-hosted projects use (Homebox, Linkwarden, Immich and Mealie all lead this way): a centered identity block, a link row, a few badges, then one polished hero image.

Stacked on #24. It uses #24's docs/screenshots/ paths. Merge #24 first, and GitHub will retarget this PR to main.

What changed

• Header: the app icon, name, tagline, and a link row (Website · Quick start · Changelog · Discussions · Report a bug).
• Badges: latest release, Docker pulls, amd64/arm64, build status and license. The build badge tracks docker-publish.yml on main, because ci.yml only runs on PRs.
• Hero image: one composite, docs/screenshots/readme-hero.png (1600×780). The three phones show the dashboard, the completed HMS Victory build and the model catalog. It replaces the three raw screenshots that sat side by side. I avoided the tall build screenshot because it shows a build marked "Planned" at 100% (the Changing Progress Resets to Planned #14 bug).
• Features: emoji-led bullets, now leading with the 10,000+ paint catalog and barcode scanning, with a privacy bullet at the end.
• Less scrolling: the environment-variable table is behind a <details> toggle ("Nothing is required" stays visible), Tech stack is folded into Local development, and there's a star call-to-action at the end.
• Cleanup: removed the screenshot crops nothing references any more. readme-mobile-build.png stays as the source for the hero.

Deliberately left out until they exist: a live demo link and a PikaPods button.

Checks

• Rendered through GitHub's Markdown API (markdown mode, the same as the repo page) with images pointed at this branch. The link row and badges each stay on one line, and the hero renders full width.
• npm run fmt:check passes.
• The images use absolute raw URLs, so the Docker Hub README sync renders them too. They resolve once this is on main.