Split OPFS tests into separate file

[Copilot]

Motivation for the change, related issues

Addresses review feedback from #2961 to split OPFS tests into a separate file to avoid an artificially large test group.

Implementation details

• Created opfs.spec.ts with all 9 OPFS-related tests (those checking browserName !== 'chromium')
• Moved saveSiteViaModal helper to the new file
• Kept serial execution mode in opfs.spec.ts since OPFS storage is shared at browser level
• Removed serial mode from website-ui.spec.ts as remaining tests don't require it

Testing Instructions (or ideally a Blueprint)

Run the E2E tests to verify both test files execute correctly:

npx playwright test packages/playground/website/playwright/e2e/opfs.spec.ts
npx playwright test packages/playground/website/playwright/e2e/website-ui.spec.ts

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[Copilot]

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