feat(manual-batch): canonical plate + manual mode as default

- canonical_plate (uppercase alnum, strips space/dot/hyphen) at all plate
  write sites; pretty_plate for render (history, XLSX, operator tile) so
  B 1234 XYZ and B1234XYZ are one plate everywhere
- group_dos_by_plate compares canonical plates -> no false mixed_plates
- warn-only (never blocking) plate format hint in operator + monitoring modals
- batch.default_mode: manual (auto merged truck loads when a sack sat in the
  counting ROI); operator banner explains plate -> start -> stop
- docs + AGENTS known-limitation note; 85 tests pass

config.yaml

@@ -123,7 +123,7 @@ batch:
   truck_gone_tolerance_seconds: 30.0 # truck-gone: in WAITING_FOR_ACTIVITY, N s with no sack crossing + no visible sack + no valid truck signal -> finalize the batch (auto-mode end condition)
   merge_threshold_seconds: 300   # deprecated by timeout_seconds and detecting sacks within the ROI
   daily_cutoff_time: "06:00"
-  default_mode: "auto"           # auto | do_manual | manual (seed when batch_mode.json missing)
+  default_mode: "manual"          # auto | do_manual | manual (seed when batch_mode.json missing). manual = operator start/stop + plate per truck (auto merges batches when a sack sits in the counting ROI)
 
 # do = Delivery Order (surat jalan). Gated manual-batch scan flow.
 # Used when batch mode == do_manual (mode itself lives in batch_mode.json).