test(demo): add registry browser smoke

[sumfxn]

What

Adds a Playwright Chromium smoke test for the built demo registry and wires it into verify, CI, and release checks.

Why

Prevents regressions around missing CSS, broken registry routes, horizontal overflow, mobile layout, console errors, and copy buttons.

Tests

• npx pnpm@10.33.2 verify
• git diff --check
• Changeset added (or N/A): N/A, test-only

Notes

• Runs against the production Next build.
• Covers desktop and mobile viewports.
• Known non-blocking noise: Browserslist/caniuse-lite warning during widget CSS build.