auto-claude: subtask-3-4 - Implement command methods (complete, uncomplete, skip)

Adds gRPC command methods to GrpcCqrsApiClient for delivery operations:
- completeDelivery: Mark delivery as completed with optional timestamp
- markDeliveryAsUncompleted: Revert completed delivery to pending
- skipDelivery: Skip a delivery that cannot be completed

All methods follow the Result<T> pattern for consistent error handling,
check CommandResponse.success flag, and map error messages appropriately.

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

lib/api/grpc_client.dart

@@ -400,6 +400,131 @@ class GrpcCqrsApiClient {
     );
   }
 
+  // ============================================================
+  // Command Methods
+  // ============================================================
+
+  /// Marks a delivery as completed.
+  ///
+  /// Returns a [Result] indicating success or failure. Optionally accepts
+  /// a [deliveredAt] timestamp; if not provided, the server will use the
+  /// current time.
+  ///
+  /// Example:
+  /// ```dart
+  /// final result = await client.completeDelivery(deliveryId: 123);
+  /// result.when(
+  ///   success: (_) => showSuccess('Delivery completed'),
+  ///   onError: (error) => showError(error.message),
+  /// );
+  /// ```
+  Future<Result<void>> completeDelivery({
+    required int deliveryId,
+    String? deliveredAt,
+  }) async {
+    final request = CompleteDeliveryRequest(
+      deliveryId: deliveryId,
+      deliveredAt: deliveredAt,
+    );
+
+    final result = await _executeWithAuth<CommandResponse>(
+      (options) => deliveryClient.completeDelivery(request, options: options),
+    );
+
+    return result.when(
+      success: (response) {
+        if (response.success) {
+          return Result.success(null);
+        }
+        return Result.error(
+          ApiError.unknown(response.message.isNotEmpty
+              ? response.message
+              : 'Failed to complete delivery'),
+        );
+      },
+      onError: (error) => Result.error(error),
+    );
+  }
+
+  /// Marks a delivery as uncompleted.
+  ///
+  /// Returns a [Result] indicating success or failure. Use this to revert
+  /// a previously completed delivery back to pending status.
+  ///
+  /// Example:
+  /// ```dart
+  /// final result = await client.markDeliveryAsUncompleted(deliveryId: 123);
+  /// result.when(
+  ///   success: (_) => showSuccess('Delivery marked as uncompleted'),
+  ///   onError: (error) => showError(error.message),
+  /// );
+  /// ```
+  Future<Result<void>> markDeliveryAsUncompleted({
+    required int deliveryId,
+  }) async {
+    final request = MarkDeliveryUncompletedRequest(
+      deliveryId: deliveryId,
+    );
+
+    final result = await _executeWithAuth<CommandResponse>(
+      (options) =>
+          deliveryClient.markDeliveryUncompleted(request, options: options),
+    );
+
+    return result.when(
+      success: (response) {
+        if (response.success) {
+          return Result.success(null);
+        }
+        return Result.error(
+          ApiError.unknown(response.message.isNotEmpty
+              ? response.message
+              : 'Failed to mark delivery as uncompleted'),
+        );
+      },
+      onError: (error) => Result.error(error),
+    );
+  }
+
+  /// Skips a delivery.
+  ///
+  /// Returns a [Result] indicating success or failure. Use this when a
+  /// delivery cannot be completed and needs to be skipped.
+  ///
+  /// Example:
+  /// ```dart
+  /// final result = await client.skipDelivery(deliveryId: 123);
+  /// result.when(
+  ///   success: (_) => showSuccess('Delivery skipped'),
+  ///   onError: (error) => showError(error.message),
+  /// );
+  /// ```
+  Future<Result<void>> skipDelivery({
+    required int deliveryId,
+  }) async {
+    final request = SkipDeliveryRequest(
+      deliveryId: deliveryId,
+    );
+
+    final result = await _executeWithAuth<CommandResponse>(
+      (options) => deliveryClient.skipDelivery(request, options: options),
+    );
+
+    return result.when(
+      success: (response) {
+        if (response.success) {
+          return Result.success(null);
+        }
+        return Result.error(
+          ApiError.unknown(response.message.isNotEmpty
+              ? response.message
+              : 'Failed to skip delivery'),
+        );
+      },
+      onError: (error) => Result.error(error),
+    );
+  }
+
   /// Shuts down the gRPC channel and releases resources.
   ///
   /// Should be called when the client is no longer needed to properly