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

docs/features/licensing.mdx

@@ -72,7 +72,7 @@ Active licenses are re-validated every **72 hours** against the Lemon Squeezy AP
 Once your license is active, you can manage your subscription in two ways:
 
 - **From the License page:** Go to **Settings > License** and click **Manage Subscription**. This opens the Lemon Squeezy billing portal where you can update your payment method, view invoices, cancel, or switch plans.
-- **From the profile menu:** Click your profile icon in the top bar and select **Billing** for quick access to the same portal.
+- **From the profile menu:** Click your profile icon in the top bar and select **Billing** for quick access to the same portal. This option is hidden for lifetime licenses.
 
 <Frame>
   <img src="/images/licensing/license-admiral-active.png" alt="License settings showing an active Admiral subscription with Manage Subscription button" />