Fix/121 user list responsiveness

This MR addresses the issue where the user list wasn't responsive on smaller screens (look at the user list within a chat session):

• Toggle button is only displayed on short screen to display the user list
• the user list is now adapted for different screen sizes

Remark: There are other pages/components in the app that are not fully responsive yet.

frontend/src/routes/sessions/[id]/+page.svelte

@@ -25,7 +25,7 @@
 			if (newShortScreen !== isShortScreen) {
 				isShortScreen = newShortScreen;
 				if (!isShortScreen) {
-					showParticipants = true; 
+					showParticipants = true;
 				}
 			}
 		}