Fix exit errors and add UI image placeholder to README

ui/app.py

@@ -42,6 +42,14 @@ class ExerciseDiaryApp(ctk.CTk):
 
         # Default View
         self.show_dashboard()
+        
+        # Handle closing
+        self.protocol("WM_DELETE_WINDOW", self.on_close)
+
+    def on_close(self):
+        self.withdraw()
+        self.quit()
+        self.destroy()
 
     def clear_main_frame(self):
         for widget in self.main_frame.winfo_children():