feat(onboarding): add welcome wizard scaffold + Step 1 (Your shop)

Lays the groundwork for the post-signup welcome wizard (Phase 2,
Task 38). Authed users hitting /welcome are routed to the next
incomplete step based on users.onboarding_step_completed +
users.onboarding_dismissed; refresh resumes correctly because every
navigation persists state server-side first.

Backend:
- Expose onboarding_step_completed (Optional[int]) and
  onboarding_dismissed (bool) on UserResponse so /auth/me drives
  client-side routing without a separate fetch.

Frontend:
- WelcomeRouter handles the /welcome decision table (dismissed → /,
  completed >=3 → /, else next step).
- WelcomeStep1 renders the "Your shop" form (company name pre-filled
  from accounts.name, team size 1-2/3-5/6-10/11-25/26+, role
  Owner/Lead Tech/Tech/Other). Continue PATCHes /users/me/onboarding-step
  with action=complete; Skip-this-step PATCHes action=skip; Skip-the-rest
  POSTs /users/me/onboarding-dismiss-rest. Each action refreshes the
  auth store before navigating so the router resumes correctly on the
  next visit.
- onboardingApi.updateStep + dismissRest (typed against backend
  OnboardingStepRequest/Response schemas).
- Routes mounted inside AppLayout so EmailVerificationBanner persists
  above each step per spec.
- 11 vitest cases covering the routing decision table + Continue / Skip
  / Skip-the-rest / persist-failure paths.

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

frontend/src/router.tsx

@@ -67,6 +67,9 @@ const DevBranchingPage = lazyWithRetry(() => import('@/pages/DevBranchingPage'))
 const GuidesHubPage = lazyWithRetry(() => import('@/pages/GuidesHubPage'))
 const GuideDetailPage = lazyWithRetry(() => import('@/pages/GuideDetailPage'))
 const AccountSettingsPage = lazyWithRetry(() => import('@/pages/AccountSettingsPage'))
+// Welcome wizard (Phase 2)
+const WelcomeRouter = lazyWithRetry(() => import('@/pages/welcome/WelcomeRouter'))
+const WelcomeStep1 = lazyWithRetry(() => import('@/pages/welcome/WelcomeStep1'))
 const NetworkDiagramsPage = lazyWithRetry(() => import('@/pages/NetworkDiagrams'))
 const DiagramEditorPage = lazyWithRetry(() => import('@/pages/NetworkDiagrams/DiagramEditor'))
 // Admin pages
@@ -240,6 +243,10 @@ export const router = sentryCreateBrowserRouter([
       { path: 'dev/branching', element: page(DevBranchingPage) },
       { path: 'guides', element: page(GuidesHubPage) },
       { path: 'guides/:slug', element: page(GuideDetailPage) },
+      // Welcome wizard (Phase 2). Mounted inside AppLayout so the email-
+      // verification banner persists above each step.
+      { path: 'welcome', element: page(WelcomeRouter) },
+      { path: 'welcome/step-1', element: page(WelcomeStep1) },
       // Admin routes
       {
         path: 'admin',