feat: adeded timer_screen and custom_timer, timer_screen connected in the main.py

kawai_focus/main.py

@@ -2,6 +2,8 @@ import kivy
 kivy.require('2.3.1')
 
 from kivy.app import App
+from kivy.uix.screenmanager import ScreenManager
+from kawai_focus.screens.timer_screen import TimerScreen
 
 
 class KawaiFocusApp(App):
@@ -10,7 +12,9 @@ class KawaiFocusApp(App):
     title = 'Kawai.Focus'
     
     def build(self):
-        pass
+        screen_manager = ScreenManager()
+        screen_manager.add_widget(TimerScreen(name='timers_screen'))
+        return screen_manager
 
 
 def main() -> None: