fix(fileloader): use untrack to prevent infinite reload loop

loadFile() reads fileUrl synchronously before its first await. When
called from a \$effect, Svelte tracks that read and re-runs the effect
every time fileUrl changes — i.e. after every successful load — causing
an infinite cycle of file fetches and PdfViewer remounts.

Fix: wrap the fileUrl read in untrack() so callers never accidentally
subscribe to fileUrl changes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/hooks/useFileLoader.svelte.ts

@@ -1,3 +1,5 @@
+import { untrack } from 'svelte';
+
 export function createFileLoader() {
 	let fileUrl = $state('');
 	let isLoading = $state(false);
@@ -6,7 +8,11 @@ export function createFileLoader() {
 	async function loadFile(url: string): Promise<void> {
 		isLoading = true;
 		fileError = '';
-		if (fileUrl) URL.revokeObjectURL(fileUrl);
+		// untrack prevents callers ($effect) from accidentally subscribing to fileUrl.
+		// Without it, the calling effect would re-run every time fileUrl changes (i.e.
+		// on every successful load), creating an infinite load loop.
+		const prev = untrack(() => fileUrl);
+		if (prev) URL.revokeObjectURL(prev);
 		fileUrl = '';
 
 		try {