test(shared-ui): add React Testing Library coverage

[luxass]

Summary

Add browser/component tests for packages/shared-ui using Vitest and React Testing Library.

Why

We are starting to lean more heavily on shared UI primitives across the monorepo, and we now have a working browser-test pattern in packages/pipelines/pipeline-server that we can reuse. A small but real test foundation in shared-ui would help us catch regressions in reusable primitives before they land in downstream apps/packages.

Proposed direction

• Add a Vitest browser/component test setup for packages/shared-ui
• Use jsdom with React Testing Library
• Keep the suite focused on behavior and accessibility rather than styling snapshots
• Follow the same general testing approach already used in pipeline-server

Suggested scope

Start small with a few high-value primitives and compositions, for example:

• ui/button
• ui/input
• ui/dialog
• ui/sidebar
• selected shared components under src/components

Expectations

• Tests should live with clear browser/component structure
• Prefer semantic queries first
• Only add data-testid when semantic queries are not stable enough
• Avoid snapshot-heavy testing
• Use React Testing Library patterns consistently

Implementation notes

• packages/shared-ui/package.json currently has no test script or Vitest config
• We can reuse the React Testing Library dependency set already proven in pipeline-server
• This is a good place to establish conventions for testing reusable UI primitives across the repo

Nice follow-up

If the setup works well, we can later use the same pattern for other reusable frontend packages too.