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/data/datasources/local_store.dart

@@ -5,6 +5,7 @@ import '../local/app_database.dart';
 import '../local/catalogue_dao.dart';
 import '../local/order_dao.dart';
 import '../local/staff_dao.dart';
+import '../local/sync_config_store.dart';
 import '../local/sync_log_dao.dart';
 import '../local/terminal_identity.dart';
 
@@ -23,6 +24,7 @@ class LocalStore {
   late OrderDao orders;
   late SyncLogDao syncLog;
   late StaffDao staff;
+  late SyncConfigStore syncConfig;
   late TerminalIdentityStore identityStore;
 
   /// Who this till is. Minted on first run, then stable forever.
@@ -52,6 +54,7 @@ class LocalStore {
     orders = OrderDao(AppDatabase.instance.db);
     syncLog = SyncLogDao(AppDatabase.instance.db);
     staff = StaffDao(AppDatabase.instance.db);
+    syncConfig = SyncConfigStore(catalogue);
     identityStore = TerminalIdentityStore(catalogue);
 
     // A terminal with no staff cannot be signed into at all, so this runs