feat: add remote products and qr code for payconic

lib/services/remote_sync_service.dart

@@ -17,6 +17,7 @@ import '../models/remote_command.dart';
 import '../utils/app_config.dart';
 import 'bar_tab_service.dart';
 import 'kooltap_api_client.dart';
+import 'product_image_sync.dart';
 
 typedef _Credentials = ({String deviceId, String token});
 
@@ -38,6 +39,10 @@ class RemoteSyncService {
   final KoolTapApiClient apiClient;
   final FlutterSecureStorage secureStorage;
 
+  /// Uploads pictures taken here before a sync and downloads new ones after;
+  /// null leaves pictures alone.
+  final ProductImageSync? imageSync;
+
   /// How long tab changes must be quiet before they are synced.
   final Duration changeDebounce;
 
@@ -49,6 +54,7 @@ class RemoteSyncService {
     required this.barTabService,
     required this.apiClient,
     FlutterSecureStorage? secureStorage,
+    this.imageSync,
     this.changeDebounce = const Duration(milliseconds: kSyncChangeDebounceMs),
     this.changeMaxDelay = const Duration(milliseconds: kSyncChangeMaxDelayMs),
   }) : secureStorage = secureStorage ?? const FlutterSecureStorage();
@@ -222,6 +228,14 @@ class RemoteSyncService {
     try {
       final appliedStock = await _doSync(credentials);
       _consecutiveFailures = 0;
+      // In the background: a slow download must not hold up the next sync
+      if (imageSync case final images?) {
+        unawaited(
+          images.downloadMissing().catchError(
+            (Object e) => debugPrint('RemoteSyncService: pictures: $e'),
+          ),
+        );
+      }
       // Show the new stock on the dashboard within seconds. The follow-up
       // skips the adjustments it already applied, so this doesn't loop.
       nextDelay = appliedStock ? Duration.zero : _pollingInterval;
@@ -278,6 +292,21 @@ class RemoteSyncService {
   /// Returns whether any stock adjustment was applied, which only happens
   /// with a server that doesn't own the stock.
   Future<bool> _doSync(_Credentials credentials) async {
+    // Pictures taken here first, so the events linking them go in this sync.
+    // One that doesn't upload (offline) waits; the sync itself still goes.
+    if (imageSync case final images?) {
+      try {
+        await images.uploadPending(
+          token: credentials.token,
+          deviceId: credentials.deviceId,
+        );
+      } on UnauthorizedException {
+        rethrow;
+      } catch (e) {
+        debugPrint('RemoteSyncService: picture upload failed: $e');
+      }
+    }
+
     final state = await _getSyncState();
     // One read, so the snapshot and the stock match the events that are sent
     final (outbox, openTabs, products, stockResults) = await database