Point the test site's code samples at the test site

[jodeleeuw]

Fixes #246.

Why

Both deploy workflows set NEXT_PUBLIC_BASE_URL, but nothing read it. The code samples on datapipe-test.web.app passed no base URL, so they fell through to the packages' built-in https://pipe.jspsych.org. A tester who copied one wrote data to production, against real researchers' experiments. The API reference also printed the production origin.

What changed

• lib/base-url.js (new) reads NEXT_PUBLIC_BASE_URL, falling back to production when it is unset or blank. It is a literal process.env reference because Next.js inlines it at build time.
• API reference: ApiPrimitives.js uses it instead of a hardcoded URL.
• Code samples: on any non-production build, every sample passes the deployment explicitly.
  • jsPsych extension: base_url in the extension params, and as an option to saveBase64Data and getCondition.
  • datapipe-client: baseURL on saveData, createSession, saveBase64Data and getCondition.
• Production samples are unchanged. There the packages' default is already right, so no override appears.

No package release is needed. @jspsych/extension-pipe@0.2.0 and datapipe-client already accept these options.

Testing

• New __tests__/base-url.test.js covers the fallback, trailing slashes, and when an override is emitted.
• extension-snippet.test.js now executes the sample with an override and checks that base_url reaches the extension.
• next build with NEXT_PUBLIC_BASE_URL=https://datapipe-test.web.app: the prerendered API reference has no pipe.jspsych.org/api, and the docs' extension samples carry the test URL.
• The JavaScript tabs are not prerendered, so a throwaway render checked them with the test URL. All four include baseURL and parse.
• eslint is clean. The full emulator suite was not run.

🤖 Generated with Claude Code