feat(chat): implement FloatingChatWidget entry point and dialog popup

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

@@ -0,0 +1,59 @@
+package com.stand.standapp.ui.chat
+
+import androidx.compose.foundation.layout.*
+import androidx.compose.foundation.shape.CircleShape
+import androidx.compose.foundation.shape.RoundedCornerShape
+import androidx.compose.material3.*
+import androidx.compose.runtime.*
+import androidx.compose.ui.Alignment
+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.unit.dp
+import androidx.compose.ui.window.Popup
+import androidx.compose.ui.window.PopupProperties
+import com.stand.standapp.ui.chat.components.ChatScaffold
+
+@Composable
+fun FloatingChatWidget() {
+    var isExpanded by remember { mutableStateOf(false) }
+
+    Box(modifier = Modifier.fillMaxSize()) {
+        // 右下角悬浮按钮
+        if (!isExpanded) {
+            FloatingActionButton(
+                onClick = { isExpanded = true },
+                modifier = Modifier
+                    .align(Alignment.BottomEnd)
+                    .padding(16.dp)
+                    .padding(bottom = 32.dp), // 留出导航栏空间
+                containerColor = Color(0xFF10B981)
+            ) {
+                Text("AI", color = Color.White, style = MaterialTheme.typography.titleMedium)
+            }
+        }
+
+        // 弹出的对话卡片
+        if (isExpanded) {
+            Popup(
+                alignment = Alignment.BottomEnd,
+                properties = PopupProperties(focusable = true)
+            ) {
+                Box(
+                    modifier = Modifier
+                        .padding(16.dp)
+                        .padding(bottom = 32.dp)
+                        .width(340.dp) // 限定卡片宽度
+                        .height(500.dp) // 限定卡片高度
+                        .shadow(8.dp, RoundedCornerShape(16.dp))
+                        .clip(RoundedCornerShape(16.dp))
+                ) {
+                    ChatScaffold(
+                        onClose = { isExpanded = false }
+                    )
+                }
+            }
+        }
+    }
+}