fix(routing): finish /home migration in WelcomeStep3 + VerifyEmailPage

The original public-landing routing refactor migrated WelcomeRouter,
WelcomeStep1, and WelcomeStep2 post-onboarding redirects to /home, but
left four sites still pointing at the old / + query-string destinations:

  - WelcomeStep3 `completeWizardAndExit` (Send invites)
  - WelcomeStep3 `handleSkipStep` (Skip)
  - VerifyEmailPage post-verify auto-redirect (`setTimeout`)
  - VerifyEmailPage success-state "Go to dashboard" Link

These all worked by accident because PublicLanding redirects authed
users from / to /home — so users still landed on the dashboard, but
through an unnecessary mount-and-redirect flicker, and the
`?welcome=true` / `?verified=1` query markers got dropped on the way.

Drop both query markers — neither is read anywhere in the codebase
(grepped frontend/src; the dashboard's onboarding UX is driven by
`getOnboardingStatus`, not URL state). Carrying dead URL params
just invites future "is this load-bearing?" investigations.

Test stubs in WelcomeStep3.test.tsx and VerifyEmailPage.test.tsx
moved from `<Route path="/">` to `<Route path="/home">` so the
assertions verify the new destination instead of accidentally matching
the old one (the previous stubs masked the partial migration).

Out of scope: AcceptInvitePage and OAuthCallbackPage still use
`?welcome=teammate`, but that one carries an explicit "decoded by the
dashboard in Task 41" annotation and may be wired up later, so left
untouched.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/pages/welcome/__tests__/WelcomeStep3.test.tsx

@@ -88,7 +88,7 @@ function renderPage() {
     <MemoryRouter initialEntries={['/welcome/step-3']}>
       <Routes>
         <Route path="/welcome/step-3" element={<WelcomeStep3 />} />
-        <Route path="/" element={<div>dashboard</div>} />
+        <Route path="/home" element={<div>dashboard</div>} />
       </Routes>
     </MemoryRouter>,
   )