fix(GRO-2099): show loading state during CustomerPortal SSO bridge bootstrap

Root cause: `Dashboard.tsx:194` runs its own `!sessionId && !isImpersonating &&
!getDevUser()` auth guard, redirecting to `/login` if `sessionId` is null. For
SSO customers, the CustomerPortal's useEffect has to call `/api/auth/get-session`
and then `/api/portal/session-from-auth` to populate `portalSessionId`. During
that bootstrap window (typically 100-300ms), `sessionId` is null and the guard
fires — redirecting the user to `/login` and breaking the post-sign-in flow.
App.tsx additionally returned `null` at `/login` for authenticated users
(`showCustomerPortal` is false at `/login`), leaving a blank React root even
if the redirect target was /login itself.

Fix:
- `CustomerPortal.tsx`: show a 'Loading…' state (`role=status`) while
  `!initComplete`. The portal chrome and its child sections only mount once
  the bootstrap has resolved, so child auth guards don't fire prematurely.
- `App.tsx`: at `/login` with a valid session, redirect to `/` so the
  customer lands on the portal instead of seeing a blank page.
- `App.tsx`: only return `LoginPage` when at `/login` — other portal
  routes defer the auth check to `CustomerPortal` (the customer SSO bridge
  resolves `portalSessionId` on mount).
- `UAT_PLAYBOOK.md`: add §5.27 with 8 cases covering the bug, the loading
  state, the /login auto-redirect, the unauth fallback, and the groomer /
  impersonation non-regressions.
- `src/__tests__/portal.test.tsx`: add a regression test that asserts the
  loading state is shown during the bridge and the portal nav is NOT in the
  DOM mid-bootstrap.

Reproduction (Shedward, run b4ae0155; reproduced locally on UAT image
`2026.06.01-ec29f71`):
1. From `about:blank`, complete customer SSO as `uat-customer`.
2. `browser_navigate` to `/portal`.
3. Pre-fix: redirected to `/login` with blank React root.
4. Post-fix: URL stays at `/portal`, dashboard renders with customer name.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

src/App.tsx

@@ -378,8 +378,12 @@ export function App() {
     return <Navigate to="/login" replace />;
   }
 
-  // Show login BEFORE checking needsSetup (needsSetup is never set for unauthenticated users)
-  if (!authDisabled && !session) {
+  // Show login BEFORE checking needsSetup (needsSetup is never set for unauthenticated users).
+  // At /login with a valid session, fall through so the staff redirect below can
+  // route staff to /admin and the final render can redirect customers to / (portal).
+  // Previously, an authenticated customer at /login would see a blank page because
+  // the final render returns null at /login (showCustomerPortal is false). See GRO-2099.
+  if (!authDisabled && !session && location.pathname === "/login") {
     return <LoginPage />;
   }
 
@@ -401,6 +405,14 @@ export function App() {
   // Don't render portal chrome at /login — DevLoginSelector is shown instead
   const showCustomerPortal = !location.pathname.startsWith("/admin") && location.pathname !== "/login";
 
+  // At /login with a valid session, redirect to the portal root. Without this,
+  // the final render returns null at /login (showCustomerPortal is false) and
+  // the user sees a blank page after a successful sign-in. Staff are routed
+  // to /admin by the earlier staff check. See GRO-2099.
+  if (!authDisabled && session && location.pathname === "/login") {
+    return <Navigate to="/" replace />;
+  }
+
   return (
     <BrandingProvider>
       {location.pathname.startsWith("/admin") ? (