feat: Phase 6 — observability dashboard (React + Vite)

[MANVENDRA-github]

What

ROADMAP Phase 6 — the dashboard. A read-only React + Vite UI (packages/dashboard) over the gateway's admin trace API.

Visualizes, all from GET /traces + GET /regression (admin-key gated, aggregated client-side):

• request volume over time, error / cache-hit / fallback rates, latency p95, total tokens, avg judge score
• distributions by provider, model, status class, and guardrail outcome; a 1–5 judge-score histogram
• a recent-requests table and the quality-regression groups (judge score by prompt × model)

How it's built

• New @sentinel/dashboard workspace: React 19 + Vite + strict TS, minimal deps (hand-rolled SVG/CSS charts — no chart library).
• Pure aggregation (aggregate.ts) + typed API client (api.ts) are unit-tested in Node (9 tests, run under the existing Vitest suite).
• The React UI is proven by a hermetic Playwright E2E — it builds the app, serves the static preview, stubs the gateway API with seeded traces, and asserts the views render. No live gateway needed.
• Tooling: scoped the gateway's strict 90% coverage + typecheck gate to packages/gateway (so the frontend coexists), added a .tsx ESLint block, a dashboard tsconfig (DOM/JSX, Bundler resolution), and a separate CI e2e job (installs Chromium, runs pnpm test:e2e).

Verify

pnpm verify        # typecheck (gateway + dashboard) + lint + 199 tests + coverage gate
pnpm test:e2e      # builds + serves the dashboard, runs Playwright (Chromium)
pnpm dev:dashboard # http://localhost:5173 against a local gateway

Verified locally: verify green (199 tests), test:e2e green (Chromium).

Notes

• The dashboard mirrors the gateway's TraceRecord type locally (small, deliberate) so the UI never depends on the gateway's build.
• Dashboard logic is unit-tested; its coverage isn't part of the gateway's 90% gate — the UI is covered by the E2E instead. A pragmatic split that keeps React/JSX out of the Node coverage gate.