feat(viewer): focus a selected card's neighbours and peek at a type from the legend

[hyamero]

What changed

• Selection focuses its path. Clicking a card keeps it, its direct neighbours and the connections between them lit, and dims the rest. Trace still widens this to everything upstream and downstream; a route still replaces it.
• Legend peek. Hovering a type in the inspector's legend dims every other type until the pointer leaves. It's transient and never enters the URL.
• No repeated title. The title card on the canvas is gone, in the viewer and in the site's embedded viewer (the component is deleted). The diagram's icon moves into the header, before the title.
• H hides the toolbar anywhere, not only in focus mode, and the toolbar's hide button is always there. On phone widths it is hidden, as Focus already is, so the toolbar stays on one row.
• Inspector layout. The gap between stage and inspector shrinks from 16px to 12px. When collapsed, the inspector is a compact square at the top instead of a full-height capsule.

Docs

• Viewer docs: the focus/H paragraph and the Select, Trace and Find descriptions.
• Key reference: the H entry.
• Landing: the Select blurb.
• README: the Explore line.

Tests

• Unit tests for neighbour focus, legend peek and H outside focus.
• The focus e2e now expects H to work outside focus mode.
• Visual baselines regenerated in the Playwright Linux image.

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[github-actions]

🎉 This PR is included in version 0.3.0-rc.1 🎉

The release is available on:

• v0.3.0-rc.1
• GitHub release

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[github-actions]

🎉 This PR is included in version 0.3.0 🎉

The release is available on:

• v0.3.0
• GitHub release

Your semantic-release bot 📦🚀