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

backend/src/index.ts

@@ -1073,12 +1073,12 @@ app.post('/api/license/validate', async (_req: Request, res: Response): Promise<
 
 app.get('/api/license/billing-portal', async (_req: Request, res: Response): Promise<void> => {
   try {
-    const url = await LicenseService.getInstance().getBillingPortalUrl();
-    if (!url) {
-      res.status(404).json({ error: 'No billing portal available. Ensure you have an active license.' });
+    const result = await LicenseService.getInstance().getBillingPortalUrl();
+    if ('error' in result) {
+      res.status(404).json({ error: result.error });
       return;
     }
-    res.json({ url });
+    res.json({ url: result.url });
   } catch (error) {
     console.error('[License] Billing portal error:', error);
     res.status(500).json({ error: 'Failed to retrieve billing portal URL' });