fix(bulk-upload): match error chips by filename, not by chunk position

save() was marking the first N files in a chunk as errored (where N = the
error count returned by the backend), but the backend errors are keyed by
filename. A failure for file[2] would incorrectly mark file[0] as the error.

Now builds a Set of error filenames and matches chunk entries by file.name.
Test added: save marks only the file whose filename matches the backend error.

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

frontend/src/lib/components/document/BulkDocumentEditLayout.svelte

@@ -109,10 +109,14 @@ async function save() {
 		if (!res.ok) {
 			hadErrors = true;
 			const body = await res.json().catch(() => ({ errors: [] }));
-			const errorCount = (body.errors ?? []).length;
-			for (let j = 0; j < errorCount && j < chunk.length; j++) {
-				const e = files.get(chunk[j].id);
-				if (e) files.set(chunk[j].id, { ...e, status: 'error' });
+			const errorFilenames = new Set<string>(
+				(body.errors ?? []).map((err: { filename: string }) => err.filename)
+			);
+			for (const entry of chunk) {
+				if (errorFilenames.has(entry.file.name)) {
+					const e = files.get(entry.id);
+					if (e) files.set(entry.id, { ...e, status: 'error' });
+				}
 			}
 		}
 		chunkProgress = { done: i + 1, total: chunks.length };