fix(billing): hide billing portal for lifetime licenses (#427)

Lifetime licenses have no recurring subscription, so the Lemon Squeezy
customer portal cannot generate a URL. The Manage Subscription button in
Settings already had the isLifetime guard, but the Billing button in the
profile dropdown did not, causing a confusing "No billing portal
available" error.

- Add !license.isLifetime guard to UserProfileDropdown (matches
  LicenseSection pattern)
- Move lifetime detection into getBillingPortalUrl() so the service owns
  all billing eligibility logic
- Change return type to { url } | { error } discriminated union for
  clear error propagation

frontend/src/components/UserProfileDropdown.tsx

@@ -77,7 +77,7 @@ export function UserProfileDropdown({ theme, setTheme, onOpenSettings }: UserPro
                         <Settings className="w-4 h-4 text-muted-foreground" />
                         Settings
                     </button>
-                    {license?.status === 'active' && (
+                    {license?.status === 'active' && !license?.isLifetime && (
                         <button
                             onClick={openBillingPortal}
                             disabled={billingLoading}