Fix onboarding private-domain history.replaceState loop past Safari's rate limit - #97709

Explanation of Change

On /onboarding/private-domain, two redirectors disagreed about where a validated private-domain user belongs, so they bounced that user back and forth. Each bounce is one history.replaceState; Mobile Safari caps that at 100/10s and throws SecurityError: Attempt to use history.replaceState() more than 100 times per 10 seconds. The rate limit is the symptom — the bug is the loop.

• The screen pushes them forward. BaseOnboardingPrivateDomain redirects validated users off this screen (to Workspaces if there are joinable policies, else Purpose/Employees) via forceReplace.
• The flow router pulled them back. getOnboardingInitialPath only redirected public-domain validated users away from private-domain; validated private-domain users with accessible policies fell through and got the stale initialPath (/onboarding/private-domain) returned, so useOnboardingFlowRouter reset them right back onto the screen.

Screen says "leave", router said "stay" → ping-pong → replaceState spam → Safari SecurityError.

This PR makes the two agree:

1. Primary (removes the bounce source) — OnboardingFlow.ts: when the last-visited path is /onboarding/private-domain and the account is validated, the router now resolves forward — Workspaces when there are accessible policies (mirroring the screen), otherwise Purpose/Employees — instead of returning the stale private-domain path. The existing guard only covered public-domain validated users; it now also covers validated private-domain users. Unvalidated users still stay on the screen to confirm their code.
2. Defensive (caps the blast radius) — BaseOnboardingPrivateDomain.tsx: a useRef "hasRedirected" guard so the forced-replace redirect fires at most once per mount, bounding replaceState calls even if a re-mount slips through.

Unit tests were added to tests/unit/OnboardingFlowTest.ts covering the new router branches (validated private-domain → Workspaces / Purpose / Employees, and unvalidated private-domain staying put).

Fixed Issues

$ #97473
PROPOSAL: #97473 (comment)

Tests

// TODO: The human co-author must fill out the manual tests they ran before marking this PR as "ready for review".
// Suggested flow: sign up with a private email domain, proceed onboarding to /onboarding/private-domain, validate the account, and confirm the app redirects forward cleanly (to the join-workspace / purpose step) without the URL flapping between routes and with no SecurityError / history.replaceState errors in the console.
// Note: automated unit tests for the router logic were added in tests/unit/OnboardingFlowTest.ts and pass.

• Verify that no errors appear in the JS console

Offline tests

QA Steps

// TODO: The human co-author must fill out the QA steps before marking this PR as "ready for review", or the issue title must include "[No QA]."

• Verify that no errors appear in the JS console

PR Author Checklist

• I linked the correct issue in the ### Fixed Issues section above
• I wrote clear testing steps that cover the changes made in this PR
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Screenshots/Videos