feat: 重构 Home Screen 视觉设计与消息输入组件

- 新增 Home Screen 视觉设计 token (背景、工具栏、对话区、输入框等)
- 重构首页布局为浮动式底部输入栈结构
- 新增 HomeBackgroundField、HomeFloatingHeader、HomeAttachmentStrip 组件
- 优化 MessageComposer 视觉样式为悬浮 shell 设计
- 添加相关测试用例

apps/test/features/home/ui/widgets/home_composer_test.dart

@@ -2,6 +2,7 @@ import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:lucide_icons/lucide_icons.dart';
+import 'package:social_app/core/theme/design_tokens.dart';
 import 'package:social_app/shared/widgets/message_composer.dart';
 
 Widget _buildTestApp({
@@ -50,8 +51,11 @@ void main() {
       );
 
       expect(find.byKey(messageComposerContainerKey), findsOneWidget);
+      expect(find.byKey(messageComposerShellKey), findsOneWidget);
+      expect(find.byKey(messageComposerInnerKey), findsOneWidget);
 
       final containerFinder = find.byKey(messageComposerContainerKey);
+      final shellFinder = find.byKey(messageComposerShellKey);
       final plusFinder = find.byKey(messageComposerPlusButtonKey);
       final rightFinder = find.byKey(messageComposerRightButtonKey);
 
@@ -63,6 +67,29 @@ void main() {
         find.descendant(of: containerFinder, matching: rightFinder),
         findsOneWidget,
       );
+
+      final container = tester.widget<Container>(shellFinder);
+      final decoration = container.decoration! as BoxDecoration;
+      expect(decoration.color, AppColors.homeComposerShell);
+      expect(
+        (decoration.border! as Border).top.color,
+        AppColors.homeComposerBorder,
+      );
+    });
+
+    testWidgets('recording state keeps unified floating shell', (tester) async {
+      await tester.pumpWidget(
+        _buildTestApp(
+          mode: MessageComposerMode.holdToSpeak,
+          process: MessageComposerProcess.recording,
+          hasMessage: false,
+          isWaitingAgent: false,
+        ),
+      );
+
+      expect(find.byKey(messageComposerShellKey), findsOneWidget);
+      expect(find.byKey(messageComposerInnerKey), findsOneWidget);
+      expect(find.text('松开发送'), findsOneWidget);
     });
 
     testWidgets('right action icon follows state priority', (tester) async {
@@ -85,7 +112,9 @@ void main() {
           of: find.byKey(messageComposerRightButtonKey),
           matching: find.byType(Icon),
         );
+        expect(iconFinder, findsOneWidget);
         final iconWidget = tester.widget<Icon>(iconFinder.first);
+        expect(iconWidget.icon, isNotNull);
         return iconWidget.icon!;
       }