Add optional KYC flow and service-backed account limits

frontend/packages/sxchnge_kyc/lib/src/transport/contracts.dart

@@ -0,0 +1,64 @@
+import '../models/identity.dart';
+import '../models/limits.dart';
+
+// Transport contracts for the stub. Replace with generated backend DTOs later.
+class IdentityDTO {
+  const IdentityDTO(this.status, {this.reason, this.id, this.simulated = true});
+  final String status;
+  final String? reason;
+  final String? id;
+  final bool simulated;
+}
+
+class IdentityMapper {
+  static Identity fromDTO(IdentityDTO dto) => Identity(
+    status: IdentityStatus.values.byName(dto.status),
+    reason: dto.reason == null ? null : ReviewReason.values.byName(dto.reason!),
+    submissionId: dto.id,
+    simulated: dto.simulated,
+  );
+}
+
+class LimitsDTO {
+  const LimitsDTO({
+    required this.tier,
+    required this.rows,
+    required this.simulated,
+  });
+  final String tier;
+  final List<LimitDTO> rows;
+  final bool simulated;
+}
+
+class LimitDTO {
+  const LimitDTO(
+    this.period,
+    this.currency,
+    this.basic,
+    this.verified,
+    this.current,
+    this.remaining,
+  );
+  final String period;
+  final String currency;
+  final int basic, verified, current, remaining;
+}
+
+class LimitsMapper {
+  static Limits fromDTO(LimitsDTO dto) => Limits(
+    tier: AccountTier.values.byName(dto.tier),
+    simulated: dto.simulated,
+    rows: dto.rows
+        .map(
+          (r) => VisibleLimit(
+            period: LimitPeriod.values.byName(r.period),
+            currency: r.currency,
+            basicMinor: r.basic,
+            verifiedMinor: r.verified,
+            currentMinor: r.current,
+            remainingMinor: r.remaining,
+          ),
+        )
+        .toList(),
+  );
+}