fix(#540): restore WebKit dashboard action focus

[BorisTyshkevich]

What & why

Closes #540.

Restores WebKit keyboard traversal for concealed Dashboard-tree actions by giving the active roving row explicit action tab stops. Focus entering a row or its nested actions synchronizes ownership, so dialog focus restoration continues through the correct row. The Library “Add to dashboard” action now has an explicit native tab stop too.

Checklist

• npm test passes (the per-file coverage gate is non-negotiable)
• Tests added/updated in the same change as the code
• npm run build succeeds (single-file dist/sql.html)
• Layers kept honest: pure logic in src/core/, network in src/net/ (injected fetch), DOM in src/ui/
• No new runtime dependency
• README / CHANGELOG.md ([Unreleased]) updated if behavior or the deployed surface changed
• Reconciled affected tracked work (roadmap Roadmap to 1.0.0 #68, the issue body, ADR/CHANGELOG) if this change reshaped it