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)

tests/test_do_batch.py

@@ -266,6 +266,41 @@ def test_start_stop_auto_409(dash_client):
     assert res.status_code == 409
 
 
+def test_manual_start_missing_plate_400(dash_client):
+    client, mode_path, state_path, _ = dash_client
+    office = {"Host": f"localhost:{cd_office_port()}"}
+    client.post("/api/batch/mode", json={"mode": "manual"}, headers=office)
+    res = client.post("/api/batch/start", headers={"Host": "localhost:5000"})
+    assert res.status_code == 400
+    body = res.get_json()
+    assert body["success"] is False
+    assert body["reason"] == "missing_plate"
+    assert not state_path.exists()
+
+    res = client.post("/api/batch/start", json={"plate": "   "},
+                      headers={"Host": "localhost:5000"})
+    assert res.status_code == 400
+    assert res.get_json()["reason"] == "missing_plate"
+    assert not state_path.exists()
+
+
+def test_manual_start_with_plate_normalizes_and_persists(dash_client):
+    client, mode_path, state_path, _ = dash_client
+    office = {"Host": f"localhost:{cd_office_port()}"}
+    client.post("/api/batch/mode", json={"mode": "manual"}, headers=office)
+    res = client.post("/api/batch/start", json={"plate": "  b 1234 xyz "},
+                      headers={"Host": "localhost:5000"})
+    assert res.status_code == 200
+    body = res.get_json()
+    assert body["success"] is True
+    assert body["plate"] == "B 1234 XYZ"
+    assert state_path.exists()
+    state = json.loads(state_path.read_text())
+    assert state["plate"] == "B 1234 XYZ"
+    assert state["batch_mode"] == "manual"
+    assert state["manual_control"] is True
+
+
 def test_stop_discard_gross_zero(dash_client):
     client, mode_path, state_path, _ = dash_client
     office = {"Host": f"localhost:{cd_office_port()}"}