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)

CHANGELOG.md

@@ -7,6 +7,13 @@ version tags are cut in this repo — POC stage, `main` is the release line).
 ## [Unreleased]
 
 ### Added
+- **Manual mode plate entry**: legacy `manual` batch start is now locked until a
+  plate number is entered — plate input in operator + monitoring start modals
+  (confirm button disabled while empty), `POST /api/batch/start` rejects
+  manual start with 400 `missing_plate` (strip + uppercase, no format regex);
+  plate flows through `current_batch.json` → `batches.plate` on stop, so
+  history/XLSX show it for manual batches. Operator plate tile now also shows
+  for manual batches (was `do_manual` only).
 - **DO-gated manual batch mode** (`auto` default · `do_manual` · legacy `manual`):
   smartphone photo capture → OCR draft → plate-grouped start gates → net
   expected/counted on live panel → stop soft-warn + force → discard only when