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>

pubspec.lock

@@ -137,6 +137,22 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.19.1"
+  connectivity_plus:
+    dependency: "direct main"
+    description:
+      name: connectivity_plus
+      sha256: "762c99f890ca8bf87f7337236f99edd42793843bc6c3631da294a76653a54bd0"
+      url: "https://pub.dev"
+    source: hosted
+    version: "7.3.1"
+  connectivity_plus_platform_interface:
+    dependency: transitive
+    description:
+      name: connectivity_plus_platform_interface
+      sha256: "3c09627c536d22fd24691a905cdd8b14520de69da52c7a97499c8be5284a32ed"
+      url: "https://pub.dev"
+    source: hosted
+    version: "2.1.0"
   crypto:
     dependency: transitive
     description:
@@ -145,6 +161,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.0.7"
+  dbus:
+    dependency: transitive
+    description:
+      name: dbus
+      sha256: "792974a4007974fbc5c1b5433eb2330a9db3e368c3f906253af4c007d0f49a91"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.7.13"
   equatable:
     dependency: "direct main"
     description:
@@ -153,6 +177,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.1.0"
+  event_bus:
+    dependency: transitive
+    description:
+      name: event_bus
+      sha256: "1a55e97923769c286d295240048fc180e7b0768902c3c2e869fe059aafa15304"
+      url: "https://pub.dev"
+    source: hosted
+    version: "2.0.1"
   fake_async:
     dependency: transitive
     description:
@@ -265,7 +297,7 @@ packages:
     source: hosted
     version: "2.0.2"
   http:
-    dependency: transitive
+    dependency: "direct main"
     description:
       name: http
       sha256: "87721a4a50b19c7f1d49001e51409bddc46303966ce89a65af4f4e6004896412"
@@ -384,6 +416,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.18.0"
+  mqtt_client:
+    dependency: "direct main"
+    description:
+      name: mqtt_client
+      sha256: "41c8edd3bc8efc80c1c8ebfb40081c24d12d13085faca96b9280a624eca2d893"
+      url: "https://pub.dev"
+    source: hosted
+    version: "10.11.11"
   native_toolchain_c:
     dependency: transitive
     description:
@@ -392,6 +432,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "0.19.2"
+  nm:
+    dependency: transitive
+    description:
+      name: nm
+      sha256: "2c9aae4127bdc8993206464fcc063611e0e36e72018696cd9631023a31b24254"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.5.0"
   objective_c:
     dependency: transitive
     description: