feat: 添加会话列表和搜索功能，重构聊天界面组件

features/im/components/conversation-header.tsx

@@ -0,0 +1,56 @@
+import MaterialIcons from "@expo/vector-icons/MaterialIcons";
+import { Pressable, StyleSheet, Text, View } from "react-native";
+
+type ConversationHeaderProps = {
+  totalLabel: string;
+  onPressCreate?: () => void;
+};
+
+export function ConversationHeader({
+  totalLabel,
+  onPressCreate,
+}: ConversationHeaderProps) {
+  return (
+    <View style={styles.container}>
+      <View style={styles.sideSpacer} />
+      <Text style={styles.title}>{`消息(${totalLabel})`}</Text>
+      <Pressable
+        accessibilityRole="button"
+        onPress={onPressCreate}
+        style={({ pressed }) => [styles.addButton, pressed && styles.addButtonPressed]}
+      >
+        <MaterialIcons color="#666666" name="add-circle-outline" size={36} />
+      </Pressable>
+    </View>
+  );
+}
+
+const styles = StyleSheet.create({
+  container: {
+    alignItems: "center",
+    flexDirection: "row",
+    height: 52,
+    justifyContent: "space-between",
+    paddingHorizontal: 20,
+  },
+  sideSpacer: {
+    width: 36,
+  },
+  title: {
+    color: "#242424",
+    fontSize: 18,
+    fontWeight: "700",
+    letterSpacing: -0.2,
+  },
+  addButton: {
+    alignItems: "center",
+    borderCurve: "continuous",
+    borderRadius: 18,
+    height: 36,
+    justifyContent: "center",
+    width: 36,
+  },
+  addButtonPressed: {
+    opacity: 0.7,
+  },
+});