feat(onboarding): add wizard Steps 2 (PSA) and 3 (Invite team)

Step 2 (`/welcome/step-2`): four PSA tiles (ConnectWise / Autotask /
HaloPSA / No PSA yet). Selecting a real PSA reveals a quiet inline
"Connect now" link to `/account/integrations` — credential entry is
intentionally OUT of the wizard. Continue persists `primary_psa`,
Skip advances without writing.

Step 3 (`/welcome/step-3`): up to 10 email/role rows (default 3,
"+ Add another" extends, role defaults to Tech / engineer with
Viewer alt). "Send invites and continue" filters empty rows, POSTs
`/accounts/me/invites/bulk`, then PATCHes onboarding-step
`{step:3, action:"complete"}` and navigates to `/?welcome=true`.
Per-row `failed[]` errors render inline next to the email and the
wizard does NOT auto-advance — user can fix-and-retry or click
"Continue anyway" to mark step complete. Empty + Skip / empty + Send
both advance without sending.

Adds `accountsApi.bulkInvite` and registers `/welcome/step-{2,3}`
in the router. Vitest: 5 named tests (selecting PSA persists,
Skip advances without primary_psa, valid emails create invites,
partial-failure inline error, empty + Skip no-op) + 5 incidental
coverage tests. tsc clean.

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

frontend/src/router.tsx

@@ -70,6 +70,8 @@ const AccountSettingsPage = lazyWithRetry(() => import('@/pages/AccountSettingsP
 // Welcome wizard (Phase 2)
 const WelcomeRouter = lazyWithRetry(() => import('@/pages/welcome/WelcomeRouter'))
 const WelcomeStep1 = lazyWithRetry(() => import('@/pages/welcome/WelcomeStep1'))
+const WelcomeStep2 = lazyWithRetry(() => import('@/pages/welcome/WelcomeStep2'))
+const WelcomeStep3 = lazyWithRetry(() => import('@/pages/welcome/WelcomeStep3'))
 const NetworkDiagramsPage = lazyWithRetry(() => import('@/pages/NetworkDiagrams'))
 const DiagramEditorPage = lazyWithRetry(() => import('@/pages/NetworkDiagrams/DiagramEditor'))
 // Admin pages
@@ -247,6 +249,8 @@ export const router = sentryCreateBrowserRouter([
       // verification banner persists above each step.
       { path: 'welcome', element: page(WelcomeRouter) },
       { path: 'welcome/step-1', element: page(WelcomeStep1) },
+      { path: 'welcome/step-2', element: page(WelcomeStep2) },
+      { path: 'welcome/step-3', element: page(WelcomeStep3) },
       // Admin routes
       {
         path: 'admin',