fix(i18n): replace hardcoded strings with Paraglide message keys

- error_file_upload_failed key used in enrich upload handler
- label_optional key added (de/en/es) and used in DescriptionSection divider

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

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

@@ -70,7 +70,7 @@ async function handleFile(file: File) {
 		await invalidate('app:document');
 	} catch (e) {
 		if ((e as Error).name === 'AbortError') return;
-		uploadError = 'Upload fehlgeschlagen. Bitte erneut versuchen.';
+		uploadError = m.error_file_upload_failed();
 	} finally {
 		isUploading = false;
 		abortController = null;