Add Notifications preferences (session-expiry reminder lead time)

- New Notifications screen (Account > Notifications): enable/disable expiry
  reminders + a 5-minute-increment stepper (5–60 min), default 15 min
- localReminders.scheduleExpiryReminder now reads the persisted lead time + enabled
  flag, so it's ready to fire once the start-session flow is wired

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/src/navigation/RootNavigator.tsx

@@ -19,6 +19,7 @@ import { AccountScreen } from '@/screens/AccountScreen';
 import { ProfileScreen } from '@/screens/ProfileScreen';
 import { VehiclesScreen } from '@/screens/VehiclesScreen';
 import { PaymentMethodsScreen } from '@/screens/PaymentMethodsScreen';
+import { NotificationsScreen } from '@/screens/NotificationsScreen';
 import { useTheme } from '@/theme/ThemeContext';
 import type { Zone } from 'parksmarter-client';
 
@@ -29,6 +30,7 @@ export type RootStackParamList = {
   Profile: undefined;
   Vehicles: undefined;
   PaymentMethods: undefined;
+  Notifications: undefined;
 };
 
 export type TabParamList = {
@@ -117,6 +119,11 @@ export function RootNavigator() {
             component={PaymentMethodsScreen}
             options={{ title: 'Payment methods' }}
           />
+          <Stack.Screen
+            name="Notifications"
+            component={NotificationsScreen}
+            options={{ title: 'Notifications' }}
+          />
         </Stack.Navigator>
       ) : (
         <LoginScreen />