refactor(fileloader): extract createFileLoader hook from document/enrich pages

Move blob URL lifecycle management into a reusable createFileLoader()
hook that owns revoke-before-create and revoke-on-destroy. Replace
identical inline logic in documents/[id] and enrich/[id] with the hook.

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

frontend/src/routes/enrich/[id]/+page.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
 import { enhance } from '$app/forms';
 import { onMount, onDestroy, untrack } from 'svelte';
+import { createFileLoader } from '$lib/hooks/useFileLoader.svelte';
 import { m } from '$lib/paraglide/messages.js';
 import DocumentViewer from '$lib/components/DocumentViewer.svelte';
 import WhoWhenSection from '$lib/components/document/WhoWhenSection.svelte';
@@ -11,9 +12,7 @@ let { data, form } = $props();
 const doc = $derived(data.document);
 
 // File preview state
-let fileUrl = $state('');
-let fileError = $state('');
-let isLoading = $state(false);
+const fileLoader = createFileLoader();
 
 let navHeight = $state(0);
 onMount(() => {
@@ -27,30 +26,11 @@ let activeAnnotationPage = $state<number | null>(null);
 
 $effect(() => {
 	if (doc?.id && doc?.filePath) {
-		loadFile(doc.id);
+		fileLoader.loadFile(`/api/documents/${doc.id}/file`);
 	}
 });
 
-async function loadFile(id: string) {
-	isLoading = true;
-	fileError = '';
-	if (fileUrl) URL.revokeObjectURL(fileUrl);
-	fileUrl = '';
-	try {
-		const response = await fetch(`/api/documents/${id}/file`);
-		if (!response.ok) throw new Error('Fehler');
-		const blob = await response.blob();
-		fileUrl = URL.createObjectURL(blob);
-	} catch {
-		fileError = m.doc_file_error_preview();
-	} finally {
-		isLoading = false;
-	}
-}
-
-onDestroy(() => {
-	if (fileUrl) URL.revokeObjectURL(fileUrl);
-});
+onDestroy(() => fileLoader.destroy());
 
 // Form state
 let tags = $state(untrack(() => doc.tags?.map((t: { name: string }) => t.name) ?? []));
@@ -93,9 +73,9 @@ let selectedReceivers = $state(untrack(() => doc.receivers ?? []));
 		<div class="relative flex-[6] overflow-hidden border-r border-line">
 			<DocumentViewer
 				doc={doc}
-				fileUrl={fileUrl}
-				isLoading={isLoading}
-				error={fileError}
+				fileUrl={fileLoader.fileUrl}
+				isLoading={fileLoader.isLoading}
+				error={fileLoader.fileError}
 				bind:annotateMode={annotateMode}
 				bind:activeAnnotationId={activeAnnotationId}
 				bind:activeAnnotationPage={activeAnnotationPage}