fix(auth): show Apple sign-in on Windows and Linux - #1757

Summary

• show Apple alongside the other OAuth providers on every desktop platform
• remove the now-unused renderer platform check
• verify through the current AuthenticationStep renderer harness that Apple renders with a Linux platform mock and dispatches signInWithSocial("apple")

Maintenance update

• synchronized the branch with main@81f8c71ed2d1abf9dd90dafcd9ae4a6da8432b48
• replaced the source-regex assertion with a component behavior regression test using the project's shared renderer test infrastructure
• mutation-checked the regression test: restoring the previous macOS-only gate fails at Apple sign-in should render on Linux

Testing

Run on Windows with Node.js 24.19.0:

• node --import tsx --test test/components/authenticationStep.test.js — 1 passed
• ESLint for AuthenticationStep.tsx and its renderer test — passed
• Prettier check for both PR files — passed
• cd src && tsc --noEmit — passed
• cd src && vite build — passed

The full suite was not rerun during this maintenance update. The focused behavior test, typecheck, lint/format checks, and production renderer build are current against the synchronized branch; official PR CI is expected to provide the repository-wide signal.

Closes #1748