feat(chat): add ChatBubble widget and mock data for home screen

- Add ChatBubble reusable widget for chat messages
- Add HomeMockData for chat list mock data
- Add HomeScreen widget tests
- Add AG-UI chat design and implementation plan docs
- Add friendship design docs
- Ignore backend/logs directory

apps/test/features/home/ui/screens/home_screen_test.dart

@@ -0,0 +1,52 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:lucide_icons/lucide_icons.dart';
+import 'package:social_app/features/home/ui/screens/home_screen.dart';
+import 'package:social_app/shared/widgets/chat_bubble.dart';
+
+void main() {
+  group('HomeScreen Widget Tests', () {
+    testWidgets('displays chat messages with ChatBubble', (
+      WidgetTester tester,
+    ) async {
+      await tester.pumpWidget(const MaterialApp(home: HomeScreen()));
+
+      expect(find.byType(ChatBubble), findsAtLeastNWidgets(1));
+    });
+
+    testWidgets('displays user request message', (WidgetTester tester) async {
+      await tester.pumpWidget(const MaterialApp(home: HomeScreen()));
+
+      expect(find.textContaining('明天提醒我开会'), findsOneWidget);
+    });
+
+    testWidgets('displays AI response message', (WidgetTester tester) async {
+      await tester.pumpWidget(const MaterialApp(home: HomeScreen()));
+
+      expect(find.textContaining('已为你创建日程'), findsOneWidget);
+    });
+
+    testWidgets('displays calendar schedule cards in chat flow', (
+      WidgetTester tester,
+    ) async {
+      await tester.pumpWidget(const MaterialApp(home: HomeScreen()));
+
+      expect(find.byType(ChatBubble), findsAtLeastNWidgets(2));
+    });
+
+    testWidgets('input field is present', (WidgetTester tester) async {
+      await tester.pumpWidget(const MaterialApp(home: HomeScreen()));
+
+      expect(find.byType(TextField), findsOneWidget);
+      expect(find.text('输入消息...'), findsOneWidget);
+    });
+
+    testWidgets('header icons are present', (WidgetTester tester) async {
+      await tester.pumpWidget(const MaterialApp(home: HomeScreen()));
+
+      expect(find.byIcon(LucideIcons.settings), findsOneWidget);
+      expect(find.byIcon(LucideIcons.calendar), findsOneWidget);
+      expect(find.byIcon(LucideIcons.messageSquare), findsOneWidget);
+    });
+  });
+}