perf(studio): virtualize the furniture table and throttle batch progress

- Render the table view through a virtualizer too, using a shared grid
  template so the sticky header and rows keep perfect column alignment
- Keep semantic table/row/cell elements while virtualizing
- Cap the batch item-details list to the latest 60 rows (newest first)
- Coalesce per-item progress events server-side (120ms throttle) in both
  the exact-import and clone SSE batch runners

src/lib/services/import/core/sse-batch.ts

@@ -79,9 +79,22 @@ export function runSseBatch<T>(opts: RunSseBatchOptions<T>): Response {
 							index,
 						});
 						try {
-							const result = await worker(item, index, (status) =>
-								send({ type: "item_progress", classname, status, index }),
-							);
+							// Coalesce micro-step progress ('downloading', 'converting',
+							// ...) to at most one per 120ms per item so large imports
+							// don't flood the client. Terminal states are emitted
+							// separately below.
+							let lastReportSent = 0;
+							const result = await worker(item, index, (status) => {
+								const now = Date.now();
+								if (now - lastReportSent < 120) return;
+								lastReportSent = now;
+								send({
+									type: "item_progress",
+									classname,
+									status,
+									index,
+								});
+							});
 							if (ac.signal.aborted) return;
 							if (result.ok) {
 								succeeded++;