test: stabilize parallel validation

[winetree94]

Summary

• make OAuth mocks request-keyed and harden WebAuthn/browser test setup
• run root validation under one CPU-sized worker budget and shard Playwright across isolated servers
• split Linux and Windows CI into parallel suites while preserving aggregate required checks and isolated performance shards
• remove CSS-class coupling from dialog helpers and harden standalone/E2E process coordination

Validation

• pnpm test (includes build, package tests, dist/example smoke, and full 4-shard Playwright E2E)
• pnpm biome check .
• git diff --check
• WebAuthn Chromium/Firefox retry-0 loop x20
• Playwright smoke loop x10 and focused TOTP/recovery boundary stress