fix(ci): launch Firefox headed via Vitest browser option

[PaulHax]

This PR targets the PaulHax mirror of Kitware#3435 (pr-3435) and contains only the CI Firefox WebGL fix.

What changed:

• Split Build and Test into a browser matrix so Chromium and Firefox run in separate jobs.
• Install/cache only the Playwright browser needed by each matrix lane.
• Set TEST_BROWSER from the matrix so Vitest runs one browser per job.
• Launch Firefox using the supported Vitest Browser instance headless: false option under xvfb-run.
• Pass Firefox WebGL prefs and Chromium SwiftShader args through @vitest/browser-playwright provider launchOptions, since instances[].launch is not consumed by Vitest 4.1.6.

Why:

The previous Firefox config used browser.instances[].launch.headless, but Vitest 4.1.6 does not feed that field into Playwright. It effectively launched Firefox as firefox.launch({ headless: true }), which produced the CI no webgl context failures. This branch feeds Firefox as firefox.launch({ firefoxUserPrefs, headless: false }).

Validation on #25:

• 5 total Build and Test attempts passed on the same SHA.
• Firefox passed all 5 attempts with 376 passed, 7 skipped, 0 no webgl context, and 0 target Proxy null.
• Local config import checks confirmed the Firefox and Chromium launch options resolve as intended.

[PaulHax]

Why this appears to fix the Firefox WebGL CI failure:

• The earlier ci-followup-cleanup / test: validate ci-followup-cleanup on fork #17 config used browser.instances[].launch.headless = false, but Vitest 4.1.6 does not consume instances[].launch for browser instances.
• As a result, test: validate ci-followup-cleanup on fork #17 effectively still launched Firefox through Playwright as firefox.launch({ headless: true }), and the Firefox prefs under instances[].launch.firefoxUserPrefs were ignored too.
• This PR uses browser.instances[].headless = false, which Vitest does merge into project.config.browser.headless before @vitest/browser-playwright launches the browser.
• Firefox prefs are passed through playwright({ launchOptions }), which the provider spreads into Playwright launch options before calling firefox.launch(...).

Effective launch shape:

// old effective path
firefox.launch({ headless: true })

// new effective path
firefox.launch({ firefoxUserPrefs, headless: false })

Validation so far:

• test: actual headed Firefox via Vitest instance option #25 passed 5 Build and Test attempts with Firefox at 376 passed, 7 skipped, 0 no webgl context, and 0 target Proxy null.
• This PR (fix(ci): launch Firefox headed via Vitest browser option #26) also passed Firefox with 376 passed, 7 skipped, 0 no webgl context, and 0 target Proxy null.