fix(frontend): destroy old WebsocketProvider on workspace switch in MultiplayerMenu (#9719)

Switching workspaces created a new WebsocketProvider without destroying
the old one. The leaked provider kept reconnecting, causing alternating
websocket traffic between old and new workspace rooms and flickering in
the Live Activity sidebar.

Add a disconnectWorkspace() cleanup that destroys the provider and resets
connected/awareness state, call it at the start of connectWorkspace()
before creating a new provider (matching ScriptEditor.svelte), and run it
from an onDestroy hook so the provider is torn down on unmount.

Fixes WIN-2086

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/sidebar/MultiplayerMenu.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
 	import { run } from 'svelte/legacy'
+	import { onDestroy } from 'svelte'
 
 	import { enterpriseLicense, userStore, workspaceStore, awarenessStore } from '$lib/stores'
 
@@ -23,7 +24,17 @@
 			url: $page.url.pathname
 		})
 	}
+	function disconnectWorkspace() {
+		if (wsProvider) {
+			wsProvider.destroy()
+			wsProvider = undefined
+		}
+		connected = false
+		awareness = undefined
+	}
 	async function connectWorkspace(workspace: string) {
+		disconnectWorkspace()
+
 		let token: string | undefined
 		try {
 			token = await signMultiplayerRequest(workspace)
@@ -73,6 +84,10 @@
 		$enterpriseLicense && $workspaceStore && connectWorkspace($workspaceStore)
 	})
 
+	onDestroy(() => {
+		disconnectWorkspace()
+	})
+
 	let peers = $derived(
 		Object.entries($awarenessStore ?? {}).filter(
 			([user]) => user && user !== 'undefined' && user !== 'null'