feat(chat): implement ChatScaffold container with sidebar drawer

app/src/main/java/com/stand/standapp/ui/chat/components/ChatScaffold.kt

@@ -0,0 +1,86 @@
+package com.stand.standapp.ui.chat.components
+
+import androidx.compose.foundation.clickable
+import androidx.compose.foundation.layout.*
+import androidx.compose.foundation.lazy.LazyColumn
+import androidx.compose.foundation.lazy.items
+import androidx.compose.foundation.shape.RoundedCornerShape
+import androidx.compose.material.icons.Icons
+import androidx.compose.material.icons.filled.Add
+import androidx.compose.material3.*
+import androidx.compose.runtime.*
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.unit.dp
+import androidx.lifecycle.viewmodel.compose.viewModel
+import com.stand.standapp.ui.chat.ChatViewModel
+import kotlinx.coroutines.launch
+
+@Composable
+fun ChatScaffold(
+    viewModel: ChatViewModel = viewModel(),
+    onClose: () -> Unit
+) {
+    val drawerState = rememberDrawerState(initialValue = DrawerValue.Closed)
+    val scope = rememberCoroutineScope()
+    val sessions by viewModel.sessions.collectAsState()
+    val currentSessionId by viewModel.currentSessionId.collectAsState()
+    val messages by viewModel.messages.collectAsState()
+
+    // 筛选当前会话的消息
+    val currentMessages = messages.filter { it.sessionId == currentSessionId }
+
+    ModalNavigationDrawer(
+        drawerState = drawerState,
+        drawerContent = {
+            ModalDrawerSheet(modifier = Modifier.fillMaxWidth(0.7f)) { // 抽屉宽度为 70%
+                Column(Modifier.fillMaxSize().padding(16.dp)) {
+                    Button(
+                        onClick = {
+                            viewModel.createNewSession()
+                            scope.launch { drawerState.close() }
+                        },
+                        modifier = Modifier.fillMaxWidth()
+                    ) {
+                        Icon(Icons.Default.Add, contentDescription = null)
+                        Spacer(Modifier.width(8.dp))
+                        Text("新建话题")
+                    }
+                    
+                    Spacer(Modifier.height(16.dp))
+                    
+                    LazyColumn {
+                        items(sessions) { session ->
+                            val isSelected = session.id == currentSessionId
+                            Surface(
+                                modifier = Modifier
+                                    .fillMaxWidth()
+                                    .padding(vertical = 4.dp)
+                                    .clickable {
+                                        viewModel.switchSession(session.id)
+                                        scope.launch { drawerState.close() }
+                                    },
+                                color = if (isSelected) Color(0xFFE5E7EB) else Color.Transparent,
+                                shape = RoundedCornerShape(8.dp)
+                            ) {
+                                Text(
+                                    text = session.title,
+                                    modifier = Modifier.padding(12.dp),
+                                    maxLines = 1
+                                )
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    ) {
+        ChatArea(
+            messages = currentMessages,
+            onSendMessage = { viewModel.sendMessage(it) },
+            onOpenDrawer = { scope.launch { drawerState.open() } },
+            onClose = onClose
+        )
+    }
+}