Add LoadingIndicator for Async Operations

Implemented a new LoadingIndicator widget that displays a pulsating gradient line, giving users visual feedback during asynchronous operations such as API calls. The indicator animates a gradient from white to AppColors.primaryLight and back to the grey background color, signifying the loading state.

frontend/lib/views/chat/loading_indicator.dart

@@ -0,0 +1,81 @@
+import 'package:auto_gpt_flutter_client/constants/app_colors.dart';
+import 'package:flutter/material.dart';
+
+class LoadingIndicator extends StatefulWidget {
+  final bool isLoading;
+
+  const LoadingIndicator({Key? key, required this.isLoading}) : super(key: key);
+
+  @override
+  _LoadingIndicatorState createState() => _LoadingIndicatorState();
+}
+
+class _LoadingIndicatorState extends State<LoadingIndicator>
+    with SingleTickerProviderStateMixin {
+  late AnimationController _controller;
+
+  @override
+  void initState() {
+    super.initState();
+
+    _controller = AnimationController(
+      duration: const Duration(seconds: 2),
+      vsync: this,
+    )..repeat();
+  }
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return LayoutBuilder(
+      builder: (context, constraints) {
+        double width =
+            (constraints.maxWidth >= 1000) ? 850 : constraints.maxWidth - 65;
+
+        return SizedBox(
+          width: width,
+          height: 4.0,
+          child: widget.isLoading
+              ? AnimatedBuilder(
+                  animation: _controller,
+                  builder: (context, child) {
+                    return ShaderMask(
+                      shaderCallback: (rect) {
+                        return LinearGradient(
+                          begin: Alignment.centerLeft,
+                          end: Alignment.centerRight,
+                          colors: [
+                            Colors.grey[400]!,
+                            AppColors.primaryLight,
+                            Colors.white,
+                            Colors.grey[400]!,
+                          ],
+                          stops: [
+                            _controller.value - 0.5,
+                            _controller.value - 0.25,
+                            _controller.value,
+                            _controller.value + 0.25,
+                          ],
+                        ).createShader(rect);
+                      },
+                      child: Container(
+                        width: width,
+                        height: 4.0,
+                        color: Colors.white,
+                      ),
+                    );
+                  },
+                )
+              : Container(
+                  color: Colors.grey[400],
+                ),
+        );
+      },
+    );
+  }
+}