Drain bills to the back office automatically, over MQTT or HTTP

Turns the orders table into a queue that empties itself. Bills were only
uploaded when a cashier pressed Sync at end of day; a till that was never
pressed held a day's takings indefinitely.

Drain engine (lib/data/sync/sync_engine.dart)
- Triggers on sale committed, network regained, 5-minute poll, head-office
  request, and the manual button.
- Single flight: a busy till firing a trigger per sale would otherwise have
  several passes reading the same pending rows and send every bill twice.
  A trigger arriving mid-drain is queued and replayed, so nothing is dropped.
- Exponential backoff with +/-20% jitter to a 5-minute ceiling. The jitter
  matters: a store's terminals all fail at the same instant when the line
  drops, and would retry in lockstep without it.
- Halts rather than loops on a failure retrying cannot fix (bad credential,
  refused batch). Pressing Sync clears the halt.

Transports (lib/data/remote/)
- OrderTransport interface; MQTT, HTTP and simulated implementations. The
  repository does not know which is in use.
- MQTT: QoS 1 uplink, application-level ACK correlated by batch_id on a return
  topic, retained Last Will for terminal-offline detection, downlink for
  catalogue pushes and remote sync requests.
- A broker PUBACK is never treated as acceptance. It means the broker holds
  the bytes, not that the ledger took the sale. Only ids the back office names
  are marked synced; silence leaves a bill pending.
- HTTP carries a stable idempotency key across retries of the same bills.

Retention
- Accepted bills are kept 7 days instead of deleted, so a batch the back
  office later loses can be re-sent in full. Purged after that; archived
  totals stay forever.
- forBusinessDate now reads pending rows only. A retained bill exists in both
  the orders table and day_archive, and summing both would overstate the day.

Fixes found while building this
- SyncEngine._refreshPending wrote state.copyWith(pending: await ...). Dart
  evaluates the receiver before the awaited argument, so a connectivity drop
  during the wait was silently overwritten by the stale snapshot. Caught by
  the first run of the new engine tests.
- PrinterSettingsController wrote state after four awaits with no mounted
  check, throwing "used after dispose" when Settings was left mid-load. This
  was pre-existing and reached the cashier as a red screen.

Also
- Header pill now reports real sync state: LIVE / n QUEUED / SYNCING /
  SYNC HALTED, with an explanation of where the bills are.
- Settings shows the route, last upload, next retry and retention window.
- docs/sync-contract.md states what the back office must implement, including
  the idempotency requirement that at-least-once delivery makes mandatory.

Tests: 90 -> 129 passing. New coverage for backoff shape and jitter band,
single flight, halting, ACK correlation and partial acceptance, at-least-once
duplicate handling, retention and purge, and no double-counting after a sync.
Suite run six times clean.

Not addressed: bills already synced by an older build went up overstated and
still need server-side reconciliation. Broker credentials have no Settings
editor yet.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

test/unit/persistence_test.dart

@@ -2,6 +2,7 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:nearle_pos/data/datasources/local_store.dart';
 import 'package:nearle_pos/data/datasources/remote_catalogue_source.dart';
 import 'package:nearle_pos/data/datasources/seed_data.dart';
+import 'package:nearle_pos/data/remote/simulated_order_transport.dart';
 import 'package:nearle_pos/data/repositories/customer_repository_impl.dart';
 import 'package:nearle_pos/data/repositories/product_repository_impl.dart';
 import 'package:nearle_pos/data/repositories/sync_repository_impl.dart';
@@ -270,7 +271,7 @@ void main() {
       final sync = SyncRepositoryImpl(
         store,
         RemoteCatalogueSource(isOffline: () => false),
-        RemoteOrderSink(isOffline: () => false),
+        SimulatedOrderTransport(isOffline: () => false),
       );
 
       final milk = (await products.findByBarcode('8901234500011'))!;
@@ -314,7 +315,7 @@ void main() {
       final sync = SyncRepositoryImpl(
         store,
         RemoteCatalogueSource(isOffline: () => false),
-        RemoteOrderSink(isOffline: () => false),
+        SimulatedOrderTransport(isOffline: () => false),
       );
 
       final milk = (await products.findByBarcode('8901234500011'))!;
@@ -343,7 +344,7 @@ void main() {
       final sync = SyncRepositoryImpl(
         store,
         RemoteCatalogueSource(isOffline: () => true),
-        RemoteOrderSink(isOffline: () => true),
+        SimulatedOrderTransport(isOffline: () => true),
       );
 
       await sync.importCatalogue();
@@ -371,7 +372,7 @@ void main() {
       final sync = SyncRepositoryImpl(
         store,
         RemoteCatalogueSource(isOffline: () => false),
-        RemoteOrderSink(isOffline: () => false),
+        SimulatedOrderTransport(isOffline: () => false),
       );
 
       await sell('Divya', 2); // 124
@@ -397,7 +398,7 @@ void main() {
       final sync = SyncRepositoryImpl(
         store,
         RemoteCatalogueSource(isOffline: () => false),
-        RemoteOrderSink(isOffline: () => false),
+        SimulatedOrderTransport(isOffline: () => false),
       );
 
       await sell('Divya', 2);