refactor(PostCard, PostCardGrid, HomeScreen): enhance PostCard component and remove PostCardGrid

- Introduced a new PostCardRelativeTime component for dynamic time display, improving user experience with real-time updates.
- Refactored PostCard to utilize memoization for performance optimization and prevent unnecessary re-renders.
- Removed the PostCardGrid component to streamline the codebase, consolidating functionality within the PostCard component.
- Updated HomeScreen to leverage the new PostCard features, ensuring consistent post rendering and interaction handling.

src/navigation/RootNavigator.tsx

@@ -249,7 +249,10 @@ export function RootNavigator({ isAuthenticated, isInitializing }: RootNavigator
       >
         {isAuthenticated ? (
           <>
-            <RootStack.Screen name="Main">
+            <RootStack.Screen
+              name="Main"
+              navigationKey={isMobile ? 'main-mobile' : 'main-desktop'}
+            >
               {() =>
                 isMobile ? (
                   <SimpleMobileTabNavigator />

src/navigation/SimpleMobileTabNavigator.tsx

@@ -1,18 +1,18 @@
 /**
- * 简单的移动端 Tab Navigator
+ * 简单的移动端 Tab Navigator（自定义 Tab 栏）
  *
- * 不使用 React Navigation 的 Tab Navigator，而是使用一个简单的自定义实现
- * 这样可以完全避免 React Navigation 状态恢复的问题
+ * 策略（在 EnsureSingleNavigator 限制下尽量省加载）：
+ * - 首页、消息：屏幕本身不带根级 NativeStack，首次进入后保留挂载，仅 display 隐藏 → 来回切换快。
+ * - 课表、我的：各含一个 NativeStack，同一时刻只挂载当前选中的一个，避免重复注册 Navigator。
+ *
+ * 其它可叠加的优化（按需再做，不放在本文件里）：
+ * - 数据：TanStack Query 调 staleTime、列表 prefetch、占位骨架
+ * - 时机：InteractionManager.runAfterInteractions 再拉非首屏接口
+ * - 渲染：React.memo 重列表项、避免 Tab 切换时整树不必要 setState
  */
 
 import React, { useEffect, useState, useCallback } from 'react';
-import {
-  View,
-  StyleSheet,
-  TouchableOpacity,
-  Text,
-  Dimensions,
-} from 'react-native';
+import { View, StyleSheet, TouchableOpacity, Text } from 'react-native';
 import { useSafeAreaInsets } from 'react-native-safe-area-context';
 import { MaterialCommunityIcons } from '@expo/vector-icons';
 import { createNativeStackNavigator } from '@react-navigation/native-stack';
@@ -22,13 +22,9 @@ import { useTotalUnreadCount } from '../stores';
 import { messageManager } from '../stores';
 
 // ==================== 导入屏幕组件 ====================
-import { HomeScreen, SearchScreen } from '../screens/home';
+import { HomeScreen } from '../screens/home';
 import { ScheduleScreen, CourseDetailScreen } from '../screens/schedule';
-import {
-  MessageListScreen,
-  NotificationsScreen,
-  PrivateChatInfoScreen,
-} from '../screens/message';
+import { MessageListScreen } from '../screens/message';
 import { ProfileScreen, SettingsScreen, EditProfileScreen, NotificationSettingsScreen, BlockedUsersScreen, AccountSecurityScreen } from '../screens/profile';
 
 // ==================== 类型定义 ====================
@@ -178,11 +174,10 @@ export function SimpleMobileTabNavigator() {
   const insets = useSafeAreaInsets();
   const messageUnreadCount = useTotalUnreadCount();
   const [activeTab, setActiveTab] = useState<TabName>('HomeTab');
-  const [mountedTabs, setMountedTabs] = useState<Record<TabName, boolean>>({
+  /** 无嵌套 Stack 的 Tab：保留实例，避免每次切回都整页重挂载 */
+  const [plainTabEverShown, setPlainTabEverShown] = useState({
     HomeTab: true,
     MessageTab: false,
-    ScheduleTab: false,
-    ProfileTab: false,
   });
 
   // 初始化 MessageManager
@@ -190,36 +185,13 @@ export function SimpleMobileTabNavigator() {
     messageManager.initialize();
   }, []);
 
-  // 处理 Tab 切换
   const handleTabPress = useCallback((tabName: TabName) => {
-    setMountedTabs((prev) => {
-      if (prev[tabName]) {
-        return prev;
-      }
-      return {
-        ...prev,
-        [tabName]: true,
-      };
-    });
+    if (tabName === 'HomeTab' || tabName === 'MessageTab') {
+      setPlainTabEverShown((prev) => ({ ...prev, [tabName]: true }));
+    }
     setActiveTab(tabName);
   }, []);
 
-  // 根据 Tab 名称渲染对应内容
-  const renderTabView = (tabName: TabName) => {
-    switch (tabName) {
-      case 'HomeTab':
-        return <HomeScreen />;
-      case 'MessageTab':
-        return <MessageListScreen />;
-      case 'ScheduleTab':
-        return <ScheduleStackNavigatorComponent />;
-      case 'ProfileTab':
-        return <ProfileStackNavigatorComponent />;
-      default:
-        return <HomeScreen />;
-    }
-  };
-
   // 渲染 Tab Bar 图标
   const renderTabIcon = (tabName: TabName, isActive: boolean) => {
     const iconColor = isActive ? colors.primary.main : colors.text.secondary;
@@ -284,31 +256,43 @@ export function SimpleMobileTabNavigator() {
     );
   };
 
-  // 计算底部安全距离
-  const bottomSafeArea = TAB_BAR_HEIGHT + TAB_BAR_FLOATING_MARGIN * 2 + insets.bottom;
-
   return (
     <View style={styles.container}>
-      {/* 内容区域 */}
       <View style={styles.content}>
-        {(Object.keys(mountedTabs) as TabName[]).map((tabName) => {
-          if (!mountedTabs[tabName]) {
-            return null;
-          }
-          const isActive = activeTab === tabName;
-          return (
-            <View
-              key={tabName}
-              style={[
-                styles.tabScreen,
-                !isActive && styles.tabScreenHidden,
-              ]}
-              pointerEvents={isActive ? 'auto' : 'none'}
-            >
-              {renderTabView(tabName)}
-            </View>
-          );
-        })}
+        {plainTabEverShown.HomeTab && (
+          <View
+            key="HomeTab"
+            style={[
+              styles.tabScreen,
+              activeTab !== 'HomeTab' && styles.tabScreenHidden,
+            ]}
+            pointerEvents={activeTab === 'HomeTab' ? 'auto' : 'none'}
+          >
+            <HomeScreen />
+          </View>
+        )}
+        {plainTabEverShown.MessageTab && (
+          <View
+            key="MessageTab"
+            style={[
+              styles.tabScreen,
+              activeTab !== 'MessageTab' && styles.tabScreenHidden,
+            ]}
+            pointerEvents={activeTab === 'MessageTab' ? 'auto' : 'none'}
+          >
+            <MessageListScreen />
+          </View>
+        )}
+        {activeTab === 'ScheduleTab' && (
+          <View key="ScheduleTab" style={styles.tabScreen}>
+            <ScheduleStackNavigatorComponent />
+          </View>
+        )}
+        {activeTab === 'ProfileTab' && (
+          <View key="ProfileTab" style={styles.tabScreen}>
+            <ProfileStackNavigatorComponent />
+          </View>
+        )}
       </View>
 
       {/* Tab Bar */}
@@ -361,9 +345,6 @@ const styles = StyleSheet.create({
     flex: 1,
     backgroundColor: colors.background.default,
   },
-  stackContainer: {
-    flex: 1,
-  },
   content: {
     flex: 1,
     position: 'relative',
@@ -374,9 +355,6 @@ const styles = StyleSheet.create({
   tabScreenHidden: {
     display: 'none',
   },
-  bottomSpacer: {
-    backgroundColor: 'transparent',
-  },
   tabBar: {
     position: 'absolute',
     left: TAB_BAR_MARGIN,
@@ -390,6 +368,9 @@ const styles = StyleSheet.create({
     justifyContent: 'space-around',
     paddingHorizontal: 8,
     ...shadows.lg,
+    // 叠在全屏内容之上（shadows.lg 含 elevation，需最后覆盖）
+    zIndex: 100,
+    elevation: 100,
   },
   tabItem: {
     flex: 1,