fix: FAB maintenance button navigates to correct screen (refs #13)

The mobile FAB 'Maintenance' option was navigating to the Vehicles screen
instead of the Maintenance screen. Updated handleQuickAction to navigate
to 'Maintenance' which displays MaintenanceMobileScreen.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -437,8 +437,7 @@ function App() {
         navigateToScreen('Documents', { source: 'quick-action' });
         break;
       case 'add-maintenance':
-        // Navigate to maintenance or open form (future implementation)
-        navigateToScreen('Vehicles', { source: 'quick-action' });
+        navigateToScreen('Maintenance', { source: 'quick-action' });
         break;
     }
   }, [navigateToScreen, navigateToVehicleSubScreen]);