feat: multi-subscription frontend support

- Add SubscriptionListItem and SubscriptionsListResponse types
- Add getSubscriptions/getSubscriptionById API methods
- Add optional subscriptionId parameter to all 30+ API methods
- Create /subscriptions page with subscription list cards
- Add /subscription/:subscriptionId route for per-subscription management
- Dashboard: show multi-tariff navigation block when user has 2+ subscriptions
- Subscription page: read subscriptionId from URL params, pass to all queries
- SubscriptionPurchase: accept subscriptionId from query params
- SubscriptionCardExpired: pass subscription.id to API calls
- Full backward compatibility: without subscriptionId, works as before

src/App.tsx

@@ -26,6 +26,7 @@ import OAuthCallback from './pages/OAuthCallback';
 import Dashboard from './pages/Dashboard';
 
 // User pages - lazy load
+const Subscriptions = lazy(() => import('./pages/Subscriptions'));
 const Subscription = lazy(() => import('./pages/Subscription'));
 const SubscriptionPurchase = lazy(() => import('./pages/SubscriptionPurchase'));
 const Balance = lazy(() => import('./pages/Balance'));
@@ -257,6 +258,26 @@ function App() {
             </ProtectedRoute>
           }
         />
+        <Route
+          path="/subscriptions"
+          element={
+            <ProtectedRoute>
+              <LazyPage>
+                <Subscriptions />
+              </LazyPage>
+            </ProtectedRoute>
+          }
+        />
+        <Route
+          path="/subscription/:subscriptionId"
+          element={
+            <ProtectedRoute>
+              <LazyPage>
+                <Subscription />
+              </LazyPage>
+            </ProtectedRoute>
+          }
+        />
         <Route
           path="/subscription"
           element={