refactor(chat): improve FloatingChatWidget layout safety and back button handling

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

@@ -1,5 +1,7 @@
 package com.stand.standapp.ui.chat
 
+import androidx.activity.compose.BackHandler
+import androidx.compose.foundation.background
 import androidx.compose.foundation.layout.*
 import androidx.compose.foundation.shape.CircleShape
 import androidx.compose.foundation.shape.RoundedCornerShape
@@ -10,6 +12,7 @@ import androidx.compose.ui.Modifier
 import androidx.compose.ui.draw.clip
 import androidx.compose.ui.draw.shadow
 import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.platform.LocalConfiguration
 import androidx.compose.ui.unit.dp
 import androidx.compose.ui.window.Popup
 import androidx.compose.ui.window.PopupProperties
@@ -18,6 +21,14 @@ import com.stand.standapp.ui.chat.components.ChatScaffold
 @Composable
 fun FloatingChatWidget() {
     var isExpanded by remember { mutableStateOf(false) }
+    val configuration = LocalConfiguration.current
+    val maxHeight = (configuration.screenHeightDp * 0.75f).dp
+
+    if (isExpanded) {
+        BackHandler {
+            isExpanded = false
+        }
+    }
 
     Box(modifier = Modifier.fillMaxSize()) {
         // 右下角悬浮按钮
@@ -44,9 +55,10 @@ fun FloatingChatWidget() {
                     modifier = Modifier
                         .padding(16.dp)
                         .padding(bottom = 32.dp)
-                        .width(340.dp) // 限定卡片宽度
+                        .widthIn(max = 340.dp) // 限定卡片宽度
-                        .height(500.dp) // 限定卡片高度
+                        .heightIn(max = maxHeight) // 限定卡片高度
                         .shadow(8.dp, RoundedCornerShape(16.dp))
+                        .background(MaterialTheme.colorScheme.surface, RoundedCornerShape(16.dp))
                         .clip(RoundedCornerShape(16.dp))
                 ) {
                     ChatScaffold(