[Phase 3] Add keyboard navigation and focus management for diagram

[Robinsonchiziterem]

Summary

Ensure the route diagram is fully keyboard navigable: Tab moves through nodes in left-to-right (or top-to-bottom on mobile) order, with visible focus rings on each interactive element.

Acceptance Criteria

• Tab key visits each asset node in route order
• Focus ring is visible (not outline: none) on each focused element
• Focus order matches the logical reading order (source → hops → destination)
• Passes axe-core accessibility audit with no violations

Drip Wave

Complexity: High — 200 pts
Phase: 3 — Production Hardening