feat(webui): Wave 6A — usage badge and failover timeline

[bodencrouch]

Summary

• Make ranked free failover legible on every turn: the routing chip expands into a keyboard-accessible timeline of ordered hop attempts (tier, endpoint, resolved model, outcome, hop index) including tier skip reasons from Wave 4B.
• Add a per-reply usage/latency badge (tokens only when the route exposes them, otherwise latency-only — never fabricated) plus a client-side session totals row.
• All data comes from client metadata already available: a new success-inclusive RouteTrace, SSE usage capture via stream_options.include_usage, and client-measured TTFT/total. No new proxy logging backend.

Origin: docs/brainstorms/2026-07-25-chat-ui-wave6-transparency-discovery-requirements.md (6A slice, R58–R64 + R88). Plan: docs/plans/2026-07-25-004-feat-chat-ui-wave6a-transparency-plan.md. Wave 5 remains blocked on this per the refreshed requirements.

Test plan

• cd webui && npm test (91 passing)
• cd webui && npm run build
• npx playwright test tests/e2e/failover-timeline.spec.ts
• Manual: send a message, expand the routing chip → hops listed with outcomes and skip reasons; alias note visible
• Manual: keyboard only — Tab to chip, Enter expands, focus ring visible
• Manual: route without usage metadata → badge shows latency only, no token counts
• Manual: session totals row updates across replies and resets on new chat
• Confirm Pages CI runs the new failover-timeline.spec.ts