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>

lib/presentation/modules/providers/printer_settings.dart

@@ -61,11 +61,22 @@ class PrinterSettingsController extends StateNotifier<PrinterSettings> {
     if (!store.isReady) return;
 
     final dao = store.catalogue;
+
+    // Read every value first, then assign. Written inline the four awaits still
+    // run before the assignment, so leaving Settings mid-read threw
+    // "used after dispose" — which reaches the cashier as a red screen.
+    final url = await dao.meta(MetaKeys.printerUrl);
+    final name = await dao.meta(MetaKeys.printerName);
+    final autoPrint = await dao.meta(MetaKeys.autoPrint);
+    final openDrawer = await dao.meta(MetaKeys.openDrawer);
+
+    if (!mounted) return;
+
     state = PrinterSettings(
-      printerUrl: await dao.meta(MetaKeys.printerUrl),
-      printerName: await dao.meta(MetaKeys.printerName),
-      autoPrint: (await dao.meta(MetaKeys.autoPrint)) == '1',
-      openDrawer: (await dao.meta(MetaKeys.openDrawer)) != '0',
+      printerUrl: url,
+      printerName: name,
+      autoPrint: autoPrint == '1',
+      openDrawer: openDrawer != '0',
     );
   }
 
@@ -75,12 +86,14 @@ class PrinterSettingsController extends StateNotifier<PrinterSettings> {
     if (printer == null) {
       await dao.setMeta(MetaKeys.printerUrl, '');
       await dao.setMeta(MetaKeys.printerName, '');
+      if (!mounted) return;
       state = state.copyWith(clearPrinter: true, autoPrint: false);
       return;
     }
 
     await dao.setMeta(MetaKeys.printerUrl, printer.url);
     await dao.setMeta(MetaKeys.printerName, printer.name);
+    if (!mounted) return;
     state = state.copyWith(
       printerUrl: printer.url,
       printerName: printer.name,
@@ -95,6 +108,7 @@ class PrinterSettingsController extends StateNotifier<PrinterSettings> {
         .read(localStoreProvider)
         .catalogue
         .setMeta(MetaKeys.autoPrint, value ? '1' : '0');
+    if (!mounted) return;
     state = state.copyWith(autoPrint: value);
   }
 
@@ -103,6 +117,7 @@ class PrinterSettingsController extends StateNotifier<PrinterSettings> {
         .read(localStoreProvider)
         .catalogue
         .setMeta(MetaKeys.openDrawer, value ? '1' : '0');
+    if (!mounted) return;
     state = state.copyWith(openDrawer: value);
   }
 }