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/components/Nav.tsx

@@ -5,10 +5,20 @@ export default function Nav() {
   const { logout } = useAuth();
 
   return (
-    <nav style={{ display: "flex", gap: 16, padding: "12px 24px", borderBottom: "1px solid #ccc" }}>
-      <Link to="/">Dashboard</Link>
-      <Link to="/profile">Profile</Link>
-      <button onClick={logout} style={{ marginLeft: "auto", cursor: "pointer" }}>
+    <nav style={{
+      display: "flex",
+      alignItems: "center",
+      gap: 16,
+      padding: "12px 24px",
+      borderBottom: "1px solid #ccc",
+    }}>
+      <Link to="/">Home</Link>
+      <Link to="/apps">Apps</Link>
+      <Link to="/settings">Settings</Link>
+      <button
+        onClick={logout}
+        style={{ marginLeft: "auto", cursor: "pointer" }}
+      >
         Logout
       </button>
     </nav>