Run web Vitest browser tests headless by default

[Copilot]

Summary

The web Vitest suite (vitest.web.config.ts) left browser.headless unset, so Vitest launched a visible browser window when run locally and only fell back to headless in CI. This makes the web tests open a browser window on every local run.

Changes

• Default to headless: set browser.headless based on a new BROWSER_HEADED env var, so the runner stays headless unless explicitly opted out.

browser: {
  enabled: true,
  // headless by default; set BROWSER_HEADED=true for a visible browser (debugging)
  headless: process.env.BROWSER_HEADED !== "true",
  provider: playwright(),
  instances: [{ browser }],
},

Checklist

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• A human-readable description of the changes was provided to include in CHANGELOG

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[Copilot]

Pull request overview

Updates the Vitest web/browser test configuration so local runs stay headless by default, avoiding a visible browser window unless explicitly requested for debugging.

Changes:

• Set browser.headless explicitly in vitest.web.config.ts to be headless by default.
• Add an opt-out via BROWSER_HEADED=true to run headed (visible) for debugging.