Add on-device debug logging: file capture + share (for real-session testing)

- fileLogger: append redacted API requests/responses + app events to an on-device
  log file (persists across restarts); the client's logSink now writes to it
- Diagnostics screen (Account > Diagnostics): toggle recording, Share log file
  (expo-sharing), clear. StartSession logs the session attempt/result explicitly.
- Lets a real parking session be captured while away from a computer, then shared.

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

app/src/navigation/RootNavigator.tsx

@@ -21,6 +21,7 @@ import { VehiclesScreen } from '@/screens/VehiclesScreen';
 import { PaymentMethodsScreen } from '@/screens/PaymentMethodsScreen';
 import { NotificationsScreen } from '@/screens/NotificationsScreen';
 import { StartSessionScreen } from '@/screens/StartSessionScreen';
+import { DiagnosticsScreen } from '@/screens/DiagnosticsScreen';
 import { useTheme } from '@/theme/ThemeContext';
 import type { Zone } from 'parksmarter-client';
 
@@ -33,6 +34,7 @@ export type RootStackParamList = {
   Vehicles: undefined;
   PaymentMethods: undefined;
   Notifications: undefined;
+  Diagnostics: undefined;
 };
 
 export type TabParamList = {
@@ -131,6 +133,11 @@ export function RootNavigator() {
             component={NotificationsScreen}
             options={{ title: 'Notifications' }}
           />
+          <Stack.Screen
+            name="Diagnostics"
+            component={DiagnosticsScreen}
+            options={{ title: 'Diagnostics' }}
+          />
         </Stack.Navigator>
       ) : (
         <LoginScreen />