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/lib/hooks/useFileLoader.svelte.ts

@@ -0,0 +1,41 @@
+export function createFileLoader() {
+	let fileUrl = $state('');
+	let isLoading = $state(false);
+	let fileError = $state('');
+
+	async function loadFile(url: string): Promise<void> {
+		isLoading = true;
+		fileError = '';
+		if (fileUrl) URL.revokeObjectURL(fileUrl);
+		fileUrl = '';
+
+		try {
+			const response = await fetch(url);
+			if (!response.ok) throw new Error('Failed to load file');
+			const blob = await response.blob();
+			fileUrl = URL.createObjectURL(blob);
+		} catch {
+			fileError = 'Vorschau konnte nicht geladen werden.';
+		} finally {
+			isLoading = false;
+		}
+	}
+
+	function destroy(): void {
+		if (fileUrl) URL.revokeObjectURL(fileUrl);
+	}
+
+	return {
+		get fileUrl() {
+			return fileUrl;
+		},
+		get isLoading() {
+			return isLoading;
+		},
+		get fileError() {
+			return fileError;
+		},
+		loadFile,
+		destroy
+	};
+}