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/documents/[id]/+page.svelte

@@ -9,6 +9,7 @@ import TranscriptionPanelHeader from '$lib/components/TranscriptionPanelHeader.s
 import type { TranscriptionBlockData } from '$lib/types';
 import { getErrorMessage } from '$lib/errors';
 import { translateOcrProgress } from '$lib/ocr/translateOcrProgress';
+import { createFileLoader } from '$lib/hooks/useFileLoader.svelte';
 
 let { data } = $props();
 
@@ -18,43 +19,15 @@ const currentUserId = $derived((data.user?.id as string | undefined) ?? null);
 
 // ── File loading ──────────────────────────────────────────────────────────────
 
-let fileUrl = $state('');
-let isLoading = $state(false);
-let fileError = $state('');
+const fileLoader = createFileLoader();
 
 $effect(() => {
 	if (doc?.id && doc?.filePath) {
-		loadFile(doc.id);
+		fileLoader.loadFile(`/api/documents/${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) {
-			if (response.status === 401) throw new Error('Nicht eingeloggt');
-			throw new Error('Fehler beim Laden der Datei');
-		}
-
-		const blob = await response.blob();
-		fileUrl = URL.createObjectURL(blob);
-	} catch (e) {
-		console.error(e);
-		fileError = 'Vorschau konnte nicht geladen werden.';
-	} finally {
-		isLoading = false;
-	}
-}
-
-onDestroy(() => {
-	if (fileUrl) URL.revokeObjectURL(fileUrl);
-});
+onDestroy(() => fileLoader.destroy());
 
 // ── Mode state ───────────────────────────────────────────────────────────────
 
@@ -350,7 +323,7 @@ onMount(() => {
 	<DocumentTopBar
 		doc={doc}
 		canWrite={canWrite}
-		fileUrl={fileUrl}
+		fileUrl={fileLoader.fileUrl}
 		bind:transcribeMode={transcribeMode}
 	/>
 
@@ -362,9 +335,9 @@ onMount(() => {
 		>
 			<DocumentViewer
 				doc={doc}
-				fileUrl={fileUrl}
-				isLoading={isLoading}
-				error={fileError}
+				fileUrl={fileLoader.fileUrl}
+				isLoading={fileLoader.isLoading}
+				error={fileLoader.fileError}
 				transcribeMode={transcribeMode && !ocrRunning}
 				blockNumbers={blockNumbers}
 				annotationReloadKey={annotationReloadKey}