T2: Embedded tooling UI baseline (Open Props + screenshot regen)

[flyingrobots]

Context

• We want embedded dashboards that require no web build step: run a binary, open a page.
• Tooling UIs should share a token-based CSS baseline and keep docs screenshots honest via Playwright.

Goal

• Establish the baseline embedded UI theme/asset pattern so future tooling dashboards are consistent and offline-friendly.

Scope

• Vendor Open Props CSS assets for embedded dashboards and serve them from binaries (offline).
• Ensure the session dashboard uses the baseline tokens (cards/nav/modals/toasts, light/dark).
• Ensure Playwright can regenerate a dashboard screenshot for docs.

Exit Criteria

• pnpm exec playwright test green.
• Embedded dashboard uses vendored tokens and the theme is consistent.
• Docs include instructions to regenerate screenshots.

Links

• docs/guide/wvp-demo.md
• crates/echo-session-ws-gateway/assets/dashboard.html
• e2e/session-dashboard.spec.ts