feat(auth): open sign-in and registration as modals over the page behind (#50)

/login, /register and /forgot-password rendered bare cards with no site header. They linked to each other and nowhere else, so a customer who clicked Log in from the storefront and changed their mind had no way back except the browser's back button. All four auth routes are now modals over the page the customer was already on, reusing the backdrop-location arrangement from #51: a direct visit or a link from an email opens over the storefront, so closing always lands somewhere real. They remain real routes, because /reset-password links to /login and customers may have bookmarks.

Signing in or registering now returns the customer to the page behind, signed in, rather than moving them to /account. Someone who signs in while browsing wants to carry on browsing, and this is already how the cart and favorites prompts behave when they resume an interrupted action.

The larger half of this is removing the duplication. Signing in existed twice — as these routes and again inside the prompt shown when a signed-out visitor adds to the cart or favorites something — and the two had already drifted. There were three different wordings of the marketing consent in circulation: the register page's, a shorter one in the prompt, and the string the server actually stores. The server keeps that text verbatim so the consent record says what the customer saw, which none of the three did. Both callers now render one shared AuthForm whose checkbox is the exact string the server records, and a test asserts that wording so it cannot drift again silently.

Steps within the auth flow replace rather than push, so switching between tabs or stepping to password recovery leaves the whole detour as a single history entry and closing returns to where it started instead of walking back through every tab that was looked at.

The privacy policy link opens in a new tab: following it in place would discard a part-filled signup form, and /privacy still has no way back of its own until #52.

Test changes follow from the destination change rather than being incidental. Nineteen assertions across seven specs waited for /account after signing in; they now assert the header shows a signed-in customer, which is the condition actually being waited for. Modal submits are scoped to their dialog, because the storefront behind now offers a Log in button of its own and an unscoped locator matched both. Assertions that follow a server round-trip were given a realistic timeout — the 5s default is too tight for a bcrypt hash plus re-rendering the storefront behind the modal.

Verified with 83 end-to-end tests, all passing, and type checking clean. No backend changes.

Closes #50

frontend/src/customer/AuthRouteModal.tsx

@@ -0,0 +1,42 @@
+import Modal from 'antd/es/modal';
+import AuthForm, { AuthMode } from './AuthForm';
+
+interface Props {
+  mode: AuthMode;
+  onClose: () => void;
+  // Moving between the auth routes, supplied by the router so the rule about
+  // keeping the whole detour to one history entry lives in one place.
+  onNavigate: (path: string) => void;
+}
+
+const TITLES: Record<AuthMode, string> = {
+  register: 'Create an account',
+  login: 'Log in'
+};
+
+// /login and /register as modals over the page behind them, so a customer who
+// clicks Log in while browsing and changes their mind is not stranded. Both
+// stay real routes: /reset-password links to /login, and customers may have
+// bookmarks.
+export default function AuthRouteModal({ mode, onClose, onNavigate }: Props) {
+  return (
+    <Modal
+      title={TITLES[mode]}
+      open
+      onCancel={onClose}
+      footer={null}
+      style={{ maxWidth: 'calc(100vw - 32px)' }}
+      destroyOnHidden
+    >
+      <AuthForm
+        mode={mode}
+        onModeChange={(next) => onNavigate(next === 'login' ? '/login' : '/register')}
+        onForgotPassword={() => onNavigate('/forgot-password')}
+        // Closing returns to the page behind, now signed in. Someone who signs
+        // in while browsing wants to carry on browsing rather than be moved to
+        // their account page.
+        onSuccess={onClose}
+      />
+    </Modal>
+  );
+}