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/__tests__/portal.test.tsx

@@ -558,4 +558,58 @@ describe("CustomerPortal SSO bridge", () => {
     expect(lastProps!.sessionId).toBe("sso-sess-1");
     expect(lastProps!.appointment.id).toBe("appt-1");
   });
+
+  // GRO-2099 regression: the portal chrome (and Dashboard's `!sessionId` guard)
+  // must NOT render before the SSO bridge resolves. A loading state must be
+  // shown instead. Previously, the Dashboard's redirect-to-/login guard fired
+  // mid-bootstrap, leaving the user with a blank page after sign-in.
+  it("renders a loading state during the SSO bridge (does not flash portal chrome)", async () => {
+    // Slow bridge: resolve get-session and session-from-auth after a tick so
+    // we can observe the loading state mid-bootstrap.
+    let resolveBridge!: (value: Response) => void;
+    const bridgePromise = new Promise<Response>((resolve) => {
+      resolveBridge = resolve;
+    });
+
+    global.fetch = vi.fn((input: RequestInfo, init?: RequestInit) => {
+      const url = typeof input === "string" ? input : input.toString();
+      if (url === "/api/branding") return Promise.resolve(brandingResponse);
+      if (url === "/api/auth/get-session") {
+        return Promise.resolve({
+          ok: true,
+          json: async () => ({ user: { email: "customer@example.com", role: "customer" } }),
+        } as Response);
+      }
+      if (url === "/api/portal/session-from-auth" && init?.method === "POST") {
+        return bridgePromise;
+      }
+      return Promise.resolve({ ok: true, json: async () => ({}) } as Response);
+    }) as unknown as typeof fetch;
+
+    const { CustomerPortal } = await import("../portal/CustomerPortal.js");
+    render(
+      <MemoryRouter initialEntries={["/"]}>
+        <CustomerPortal />
+      </MemoryRouter>
+    );
+
+    // Loading state is visible while the bridge is in flight. The portal nav
+    // (Home / Appointments / etc.) must NOT be present — its presence would
+    // indicate the chrome is rendering with a null session, which is the
+    // pre-GRO-2099 bug.
+    expect(await screen.findByRole("status")).toHaveTextContent(/Loading/i);
+    expect(screen.queryByText("Home")).not.toBeInTheDocument();
+    expect(screen.queryByText("Appointments")).not.toBeInTheDocument();
+
+    // Resolve the bridge and confirm the portal renders normally.
+    resolveBridge({
+      ok: true,
+      status: 201,
+      json: async () => ({ sessionId: "sso-sess-1", clientId: "client-1", clientName: "Jane Doe" }),
+    } as Response);
+
+    await waitFor(() => {
+      expect(screen.getByText(/Hi, Jane/)).toBeInTheDocument();
+    });
+  });
 });