feat: more easter eggs + version checking

lib/services/remote_sync_service.dart

@@ -273,6 +273,13 @@ class RemoteSyncService {
       stopPeriodicSync();
       _emit(RemoteSyncEventType.unauthorized);
       return;
+    } on UpdateRequiredException catch (e) {
+      // Retrying can't help until the app is updated. The token and every
+      // outbox stay, so nothing is lost; a manual sync or restart tries again
+      debugPrint('RemoteSyncService: app update required');
+      stopPeriodicSync();
+      _emit(RemoteSyncEventType.updateRequired, message: e.message);
+      return;
     } on RateLimitedException catch (e) {
       nextDelay = await _recordFailure(e, retryAfter: e.retryAfter);
     } catch (e, stack) {
@@ -328,6 +335,8 @@ class RemoteSyncService {
         );
       } on UnauthorizedException {
         rethrow;
+      } on UpdateRequiredException {
+        rethrow;
       } catch (e) {
         debugPrint('RemoteSyncService: picture upload failed: $e');
       }