Open the cash drawer for real, and persist the back-office route

Cash drawer
- openCashDrawer was a debugPrint. The drawer never opened.
- It cannot go through the PDF pipeline: a PDF is rendered by the platform
  driver, which will not pass raw ESC/POS bytes to the device. So it goes over
  a socket instead — nearly every network thermal printer listens on 9100 and
  forwards whatever arrives straight to the print head, which makes the whole
  protocol five bytes.
- Printer IP and port are configurable in Settings with a Test button that
  saves and fires immediately, because a drawer that does not open is
  indistinguishable from one that is not wired up.
- Every failure explains itself: unreachable, refused, or simply not
  configured — which is the honest state for a USB printer, since there is no
  raw path to one from Flutter.
- Now fires only on a cash tender. A card-only sale that pops the drawer is a
  shrinkage risk, and it is the first thing a shop notices.

Back-office route
- Host, port, TLS and transport persist to the database; username, password
  and API key go to the platform keystore (Keychain / Credential Manager /
  Android Keystore). Writing credentials into SQLite would put them in the
  same file as the bills, on a machine behind a shop counter.
- Loaded at startup. Previously the dialog wrote settings that were silently
  ignored on the next launch, which reads exactly like they never saved — and
  credentials retyped every morning end up on a sticky note instead.
- A saved route never overwrites the terminal's store or terminal id. Those
  belong to the device, and re-pointing a till at a different broker must not
  change who it is, or its bills and presence records stop lining up.

Tests: 168 -> 176. The drawer test stands up a real socket server and asserts
the exact bytes arrive. The config test asserts no credential appears anywhere
in the meta table while the non-secret settings do.

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

lib/presentation/sync/providers/sync_controller.dart

@@ -211,6 +211,15 @@ final orderSyncProvider =
 /// Overridden to a no-op in widget tests, which have no network stack and
 /// cannot drive real disk I/O on a fake clock.
 final syncBootstrapProvider = FutureProvider<void>((ref) async {
+  // Restore the route this terminal was pointed at. Without this the settings
+  // are written on Save and then silently ignored on the next launch, which
+  // reads exactly like they never saved.
+  final store = ref.read(localStoreProvider);
+  if (store.isReady) {
+    ref.read(syncConfigProvider.notifier).state =
+        await store.syncConfig.load(ref.read(syncConfigProvider));
+  }
+
   await ref.read(connectivityServiceProvider).start();
 
   final engine = ref.read(syncEngineProvider);