Feat: add pages and navigation between them; random mass

lib/pages/gain_room.dart

@@ -0,0 +1,43 @@
+import 'package:flutter/material.dart';
+import 'dart:math';
+
+int randomInRange(int min, int max) {
+  final random = Random();
+  return min + random.nextInt(max - min + 1);
+}
+
+
+class GainRoom extends StatefulWidget {
+  const GainRoom({super.key});
+
+  @override
+  State<GainRoom> createState() => _GainRoomState();
+}
+
+
+class _GainRoomState extends State<GainRoom>{
+  //vars
+  int _mass = randomInRange(40, 80);
+  //methods
+  void _gainMass(){
+    setState(() {
+    _mass += randomInRange(1, 3);
+    });
+  }
+  //ui
+  @override
+  Widget build(BuildContext context){
+    return Scaffold(
+      appBar: AppBar(title: Text("Gain Room")),
+      body: Center(
+        child: Column(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: [
+            Text("Current mass:"),
+            Text(_mass.toString()),
+          ],
+        ),
+      ),
+    );
+  }
+}