Basic layout, navigation, first empty pages

lib/screens/calendar_screen.dart

@@ -0,0 +1,10 @@
+import 'package:flutter/material.dart';
+
+class CalendarScreen extends StatelessWidget {
+  const CalendarScreen({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const Text("Calendar");
+  }
+}