feat(adminsite): session guards and the auth screens

Route-group layouts do the guarding. A customer session on /staff/* is
redirected to its own home rather than shown a refusal -- there is nothing to
tell them about. This is UX only: admin enforces the same boundary with
RequireStaff/RequireCustomer and answers 404 rather than 403 for another
account's data, which is the layer that actually matters.

Signup carries the honeypot the backend expects and reports "check your
email" rather than claiming an account exists, matching a backend that
creates nothing until the link is opened.

Buttons match site/'s .btn--solid and .btn--line, neutral border included.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

adminsite/lib/session.test.tsx

@@ -0,0 +1,45 @@
+import { render, screen, waitFor } from "@testing-library/react";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { ApiError } from "./api";
+import { RequireKind } from "./session";
+
+const replace = vi.fn();
+vi.mock("next/navigation", () => ({
+    useRouter: () => ({ replace, push: vi.fn() }),
+}));
+
+const me = vi.fn();
+vi.mock("./api", async () => {
+    const actual = await vi.importActual<typeof import("./api")>("./api");
+    return { ...actual, api: { ...actual.api, me: () => me() } };
+});
+
+function wrap(ui: React.ReactNode) {
+    const client = new QueryClient({ defaultOptions: { queries: { retry: false } } });
+    return render(<QueryClientProvider client={client}>{ui}</QueryClientProvider>);
+}
+
+describe("RequireKind", () => {
+    beforeEach(() => replace.mockClear());
+
+    it("renders staff screens for a staff session", async () => {
+        me.mockResolvedValue({ kind: "staff", email: "s@example.com" });
+        wrap(<RequireKind kind="staff">operations</RequireKind>);
+        expect(await screen.findByText("operations")).toBeInTheDocument();
+        expect(replace).not.toHaveBeenCalled();
+    });
+
+    it("sends a customer session away from staff screens without telling it anything", async () => {
+        me.mockResolvedValue({ kind: "customer", email: "c@example.com", account_id: "a1" });
+        wrap(<RequireKind kind="staff">operations</RequireKind>);
+        await waitFor(() => expect(replace).toHaveBeenCalledWith("/"));
+        expect(screen.queryByText("operations")).not.toBeInTheDocument();
+    });
+
+    it("sends an unauthenticated visitor to sign in", async () => {
+        me.mockRejectedValue(new ApiError(401, "not signed in"));
+        wrap(<RequireKind kind="customer">account</RequireKind>);
+        await waitFor(() => expect(replace).toHaveBeenCalledWith("/login"));
+    });
+});