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

@@ -3,33 +3,48 @@
 	import type { PageData } from './$types.js';
 	import WeeklySurvey from './WeeklySurvey.svelte';
 	import Chatbox from './Chatbox.svelte';
-	import { writable } from 'svelte/store';
+	import { onMount } from 'svelte';
+
+	let showParticipants = $state(false);
+	let isShortScreen = $state(false);
 
 	let { data }: { data: PageData } = $props();
 	let user = data.user!;
 	let { session, jwt } = data;
 	let { onlineUsers } = session;
 
-	let showParticipants = writable(false);
-
 	function toggleParticipants() {
-		showParticipants.update((value) => !value);
+		showParticipants = !showParticipants;
 	}
 
-	let isShortScreen = writable(false);
+	onMount(() => {
+		isShortScreen = typeof window !== 'undefined' && window.innerWidth < 1024;
 
-	function handleResize() {
-		isShortScreen.set(window.innerWidth < 1024);
-	}
-	window.addEventListener('resize', handleResize);
-	handleResize();
+		function handleResize() {
+			const wasShortScreen = isShortScreen;
+			isShortScreen = window.innerWidth < 1024;
+
+			if (!isShortScreen && wasShortScreen) {
+				showParticipants = true;
+			}
+		}
+
+		if (typeof window !== 'undefined') {
+			window.addEventListener('resize', handleResize);
+			handleResize();
+
+			return () => {
+				window.removeEventListener('resize', handleResize);
+			};
+		}
+	});
 </script>
 
 <div class="h-full flex lg:flex-row flex-col pt-2 lg:pt-0 bg-gray-50 relative">
-	{#if $isShortScreen}
+	{#if isShortScreen}
 		<button
 			class="absolute top-4 right-4 bg-yellow-500 text-white px-2 py-1 rounded-md shadow-md hover:bg-yellow-600 transition focus:outline-none text-sm"
-			on:click={toggleParticipants}
+			onclick={toggleParticipants}
 		>
 			{$t('utils.words.toggle')}
 		</button>
@@ -37,7 +52,7 @@
 
 	<div
 		class="bg-white border rounded-lg shadow-md p-6 mx-4 my-2 h-fit w-full max-w-md text-base lg:text-lg"
-		class:!hidden={!$showParticipants && !window.matchMedia('(min-width: 1024px)').matches}
+		class:!hidden={typeof window !== 'undefined' && !showParticipants && !window.matchMedia('(min-width: 1024px)').matches}
 	>
 		<h2 class="text-xl truncate font-semibold text-gray-700 text-center mb-4">
 			{$t('utils.words.participants')}