feat(manual-batch): require plate number at manual mode start

Legacy manual batch start locked until plate entered:
- POST /api/batch/start manual branch returns 400 missing_plate when
  plate empty (strip + uppercase, no format regex)
- operator + monitoring start modals gain required plate input,
  confirm button disabled until filled
- operator plate tile also shows for manual batches (was do_manual only)
- do_manual unchanged (plate from DOs)

counter_dashboard.py

@@ -472,6 +472,17 @@ def api_batch_start():
             expected_sack = sum(int(d.get("expected_sack") or 0) for d in dos)
             expected_box = sum(int(d.get("expected_box") or 0) for d in dos)
 
+        elif mode == "manual":
+            # Legacy manual mode: batch start locked until plate entered.
+            req_data = request.get_json(silent=True) or request.form or {}
+            plate = str(req_data.get("plate", "")).strip().upper()
+            if not plate:
+                return jsonify({
+                    "success": False,
+                    "reason": "missing_plate",
+                    "error": "Plat nomor wajib diisi sebelum batch dimulai.",
+                }), 400
+
         counting_date = get_counting_date()
         batch_num = get_next_batch_number(counting_date)
         now_iso = datetime.now().isoformat()