Run web Vitest browser tests headless by default (#930)

## 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.

```ts
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

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Author: Copilot

vitest.web.config.ts

@@ -133,6 +133,11 @@ export default defineConfig({
     },
     browser: {
       enabled: true,
+      // Run headless by default so the test runner never pops up a browser
+      // window (locally Vitest would otherwise launch a headed browser; it only
+      // defaults to headless in CI). Set BROWSER_HEADED=true to opt back into a
+      // visible browser, e.g. for debugging.
+      headless: process.env.BROWSER_HEADED !== "true",
       provider: playwright(),
       instances: [{ browser }],
     },