Wire up start-session flow (variable-rate aware) + fix About dark mode

- New StartSession screen off MeterDetail: pick vehicle -> duration (from the live
  multi-estimate ladder, exact cost per option) -> card; detects a free prefix and
  shows "Free until X, then charged"; explicit Pay $X confirmation before charging
  via postStartParkingSession; schedules the local expiry reminder on success
- About screen themed (was black-on-dark in dark mode)

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

app/src/navigation/RootNavigator.tsx

@@ -20,12 +20,14 @@ import { ProfileScreen } from '@/screens/ProfileScreen';
 import { VehiclesScreen } from '@/screens/VehiclesScreen';
 import { PaymentMethodsScreen } from '@/screens/PaymentMethodsScreen';
 import { NotificationsScreen } from '@/screens/NotificationsScreen';
+import { StartSessionScreen } from '@/screens/StartSessionScreen';
 import { useTheme } from '@/theme/ThemeContext';
 import type { Zone } from 'parksmarter-client';
 
 export type RootStackParamList = {
   Tabs: undefined;
   MeterDetail: { zone: Zone };
+  StartSession: { zone: Zone };
   About: undefined;
   Profile: undefined;
   Vehicles: undefined;
@@ -111,6 +113,11 @@ export function RootNavigator() {
             component={MeterDetailScreen}
             options={{ title: 'Meter' }}
           />
+          <Stack.Screen
+            name="StartSession"
+            component={StartSessionScreen}
+            options={{ title: 'Start session' }}
+          />
           <Stack.Screen name="About" component={AboutScreen} options={{ title: 'About' }} />
           <Stack.Screen name="Profile" component={ProfileScreen} options={{ title: 'Profile' }} />
           <Stack.Screen name="Vehicles" component={VehiclesScreen} options={{ title: 'Vehicles' }} />