feat: add frontend routing analytics views for dashboard

Summary

Add dashboard frontend views for routing analytics once backend analytics APIs are available.

Parent roadmap: #2287 Parent issue: #2345 Depends on: backend analytics read model issue Related code: dashboard/frontend/src/pages, dashboard/frontend/src/components, dashboard/frontend/e2e

Motivation

Operators need to see how traffic is routed, where confidence is low, which recipes are active, and how route/model latency changes over time. This should be a dashboard-native workflow backed by normalized APIs.

Scope

Build frontend views for:

• route/model/recipe distribution;
• signal confidence and projection summaries;
• latency/overhead trends;
• low-confidence/unrouted traffic summaries;
• drill-down links to replay records;
• empty/error/loading states.

Non-goals

• Do not expose raw sensitive prompts in aggregate views.
• Do not add chart-only logic that duplicates backend aggregation.
• Do not block router runtime on dashboard availability.

Acceptance criteria

• Dashboard users can inspect routing quality and operational signals without reading raw replay records.
• Views are responsive and covered by focused frontend/e2e tests.
• Privacy and retention notes are visible in docs, not hard-coded as dashboard-only assumptions.