test: add demo video regression coverage

[kimberlydemaio]

Why this change?

Issue #68 requests a regression test for the /demo-video page so future changes don't accidentally break the video, captions, poster, or demo link.

Closes #68

What changed?

Added a new unit test for the /demo-video page.

• an HTML5 video element is rendered
• the video source points to /media/codeverse-demo.mp4
• the poster references /media/codeverse-demo-poster.jpg
• an English captions track points to /media/codeverse-demo.vtt
• the captions track is not marked as default
• the page includes a link to the interactive /demo workspace

No production code was changed.

How did you verify it?

• Relevant unit or server tests
• npm run lint
• npm run typecheck
• npm run audit:repo
• Relevant application or catalog audits
• Production build when the client changed
• Screenshots or recording for visible UI changes

Ran npx vitest run tests/unit/demo-video.test.tsx

All 6 tests passed.

Risk, rollout, and recovery

Low risk. This is a test-only change, so it shouldn't affect the application.

If needed, it can be reverted by removing the new test file.

Reviewer guide

Please review:

• client/tests/unit/demo-video.test.tsx
• whether the six assertions accurately cover the requirements in issue [First PR] Add a regression test for the narrated demo player #68
• whether the selectors and expectations match the existing testing conventions in the client test suite

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