Redesign login as landing page, remove self-registration, add nav+placeholders

- LoginPage: centred landing layout with logo placeholder box and business
  name headline (BUSINESS_NAME constant); registration link removed
- useAuth: post-login redirect goes to / (dashboard) directly
- Nav: Home | Apps | Settings | Logout (consistent on all protected pages)
- AppsPage, SettingsPage: white placeholder pages with headline
- App.tsx: /apps and /settings private routes; removed /register,
  /register-success, /login-success; catch-all → /
- Deleted: RegisterPage, RegisterSuccessPage, LoginSuccessPage
- Backend /api/auth/register kept for future admin-side user creation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/AppsPage.tsx

@@ -0,0 +1,12 @@
+import Nav from "../components/Nav";
+
+export default function AppsPage() {
+  return (
+    <>
+      <Nav />
+      <div style={{ padding: 32 }}>
+        <h1>Apps</h1>
+      </div>
+    </>
+  );
+}