added pair screen, home screen, and channels screen

desktop_app/ui/screens/main_screen.py

@@ -0,0 +1,17 @@
+from textual.screen import Screen
+from textual.widgets import Header, Footer, ContentSwitcher
+from ui.widgets.home_sidebar import HomeSidebar
+from ui.widgets.home_info import HomeInfo
+from ui.widgets.channels_list import ChannelsList
+
+
+class MainScreen(Screen):
+    def compose(self):
+        yield Header(show_clock=True)
+        yield HomeSidebar()
+        
+        with ContentSwitcher(initial="home-info"):
+            yield HomeInfo(id="home-info")
+            yield ChannelsList(id="channels-list")
+        
+        yield Footer()