chore: replace all '(demo)' labels with '(coming soon)'

Affects snackbar messages in booking_screen, tickets_booked_screen,
calendar_screen, settings_screen. Also updates Privacy Policy subtitle
from 'Demo app' to 'Coming Soon'.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/screens/tickets_booked_screen.dart

@@ -6,19 +6,19 @@ class TicketsBookedScreen extends StatelessWidget {
 
   void _onScannerTap(BuildContext context) {
     ScaffoldMessenger.of(context).showSnackBar(
-      SnackBar(content: Text('Scanner tapped (demo)')),
+      SnackBar(content: Text('Scanner tapped (coming soon)')),
     );
   }
 
   void _onWhatsappTap(BuildContext context) {
     ScaffoldMessenger.of(context).showSnackBar(
-      SnackBar(content: Text('Chat/WhatsApp tapped (demo)')),
+      SnackBar(content: Text('Chat/WhatsApp (coming soon)')),
     );
   }
 
   void _onCallTap(BuildContext context) {
     ScaffoldMessenger.of(context).showSnackBar(
-      SnackBar(content: Text('Call tapped (demo)')),
+      SnackBar(content: Text('Call (coming soon)')),
     );
   }