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/__tests__/VerifyEmailPage.test.tsx

@@ -52,7 +52,7 @@ function renderPage(initialPath: string) {
       <MemoryRouter initialEntries={[initialPath]}>
         <Routes>
           <Route path="/verify-email" element={<VerifyEmailPage />} />
-          <Route path="/" element={<div>dashboard</div>} />
+          <Route path="/home" element={<div>dashboard</div>} />
         </Routes>
       </MemoryRouter>
     </HelmetProvider>,
@@ -130,7 +130,7 @@ describe('VerifyEmailPage', () => {
         <MemoryRouter initialEntries={['/verify-email?token=valid-token']}>
           <Routes>
             <Route path="/verify-email" element={<VerifyEmailPage />} />
-            <Route path="/" element={<div>dashboard</div>} />
+            <Route path="/home" element={<div>dashboard</div>} />
           </Routes>
         </MemoryRouter>
       </HelmetProvider>,
@@ -142,7 +142,7 @@ describe('VerifyEmailPage', () => {
         <MemoryRouter initialEntries={['/verify-email?token=valid-token']}>
           <Routes>
             <Route path="/verify-email" element={<VerifyEmailPage />} />
-            <Route path="/" element={<div>dashboard</div>} />
+            <Route path="/home" element={<div>dashboard</div>} />
           </Routes>
         </MemoryRouter>
       </HelmetProvider>,