Fix navigation state handling in layout component

- Updated the `isNavigating` derived variable to use a boolean conversion instead of a strict null check, preventing the navigation indicator from hanging when `navigating` is undefined.
- Added a comment to clarify the reason for this change, enhancing code readability and maintainability.

web/src/routes/+layout.svelte

@@ -15,7 +15,8 @@
 	let { children } = $props();
 
 	const docTitle = $derived(`${panelTitleFromPath(page.url.pathname)} · Telemt Panel`);
-	const isNavigating = $derived(navigating !== null);
+	/** Нельзя использовать `!== null`: при `navigating === undefined` выражение истинно и полоска «зависает». */
+	const isNavigating = $derived(!!navigating);
 
 	let serverAliases = $state<string[]>([]);