add onload & fallback rendering

fixes API trying to initialize before it has loaded

src/lib/Jitsi.svelte

@@ -1,5 +1,15 @@
+<!-- @component
+The Jitsi component is a lightweight wrapper that manages loading the Jitsi Meet External API
+and joining a Jitsi meeting room. It accepts configuration options and provides a callback
+when the API is ready for use.
+
+WARNING: Assume that ALL properties are NOT reactive. Due to the nature of the Jitsi Meet
+External API, changing properties after initialization will NOT update the Jitsi instance.
+Instead, use the getAPI() method to interact with the Jitsi instance directly.
+-->
+
 <script lang="ts">
-	import { onMount } from 'svelte';
+	import { onMount, type Snippet } from 'svelte';
 	import type {
 		JitsiMeetExternalAPIOptions,
 		JitsiMeetExternalAPI
@@ -21,13 +31,30 @@
 		 * Additional CSS classes to apply to the Jitsi container.
 		 */
 		class?: ClassValue;
+		/**
+		 * Children are rendered inside the Jitsi container until the API is initialized.
+		 */
+		children?: Snippet;
+		/**
+		 * Callback function that is called when the Jitsi API is loaded. Can
+		 * be used to provide a loading indicator until the API is ready.
+		 * @param api The Jitsi Meet External API instance.
+		 */
+		onready?: (api: JitsiMeetExternalAPI) => void;
 	}
 
-	let { domain = 'https://meet.jit.si', options, class: classValue }: Props = $props();
+	let {
+		domain = 'https://meet.jit.si',
+		options,
+		class: classValue,
+		children,
+		onready
+	}: Props = $props();
 
 	const src = $derived(`${new URL('external_api.js', domain).toString()}`);
 
 	let container: HTMLDivElement;
+	let ready = $state(false);
 	let api = $state<JitsiMeetExternalAPI | null>(null);
 
 	onMount(() => {
@@ -35,13 +62,27 @@
 		if (document.querySelectorAll('#jitsi-container').length > 1) {
 			console.warn('Duplicate Jitsi container detected. This may lead to unexpected behavior.');
 		}
+	});
 
+	const onload = () => {
+		if (typeof window.JitsiMeetExternalAPI === 'undefined') {
+			console.error('Jitsi Meet External API is not available on the window object.');
+			return;
+		}
+
+		ready = true;
 		// Initialize the Jitsi Meet External API
 		api = new window.JitsiMeetExternalAPI(new URL(domain).host, {
 			...options,
 			parentNode: container
 		});
-	});
+		if (api === null) {
+			console.error('Failed to initialize Jitsi Meet External API.');
+			return;
+		}
+		// Call the onready callback if provided
+		onready?.(api);
+	};
 
 	/**
 	 * Get the Jitsi Meet External API instance.
@@ -53,7 +94,11 @@
 </script>
 
 <svelte:head>
-	<script {src}></script>
+	<script {src} {onload}></script>
 </svelte:head>
 
-<div id="jitsi-container" bind:this={container} class={[classValue]}></div>
+<div id="jitsi-container" bind:this={container} class={[classValue]}>
+	{#if !ready}
+		{@render children?.()}
+	{/if}
+</div>