perf(home): 四个根 tab 改为首次可见才构建

原先 IndexedStack 冷启动就把推荐/发现/追剧/我的四棵子树全建出来，各自还会发请求；
现在只建当前 tab，切过去再建并常驻，冷启动少三页构建和一次观看历史请求。

lib/pages/home/home_view.dart

@@ -161,24 +161,18 @@ class _HomeTabsState extends State<_HomeTabs> {
   }
 
   Widget _body(BuildContext context) {
-    final pages = <Widget>[
-      _recommend(context),
-      _recommendDiscover(context),
-      FollowingPage(
+    final builders = <WidgetBuilder>[
+      _recommend,
+      _recommendDiscover,
+      (context) => FollowingPage(
         onGoHome: () => logic.select(0),
         favoriteController: context.read<FavoriteController>(),
         historyRequest: logic.followingHistoryRequest,
         visible: logic.tab == 2,
       ),
-      _my(context),
+      _my,
     ];
-    return IndexedStack(
-      index: logic.tab,
-      children: [
-        for (var index = 0; index < pages.length; index++)
-          TickerMode(enabled: logic.tab == index, child: pages[index]),
-      ],
-    );
+    return _LazyTabStack(index: logic.tab, builders: builders);
   }
 
   Widget _recommend(BuildContext context) {
@@ -248,3 +242,42 @@ class _HomeTabsState extends State<_HomeTabs> {
     logic.select(tab);
   }
 }
+
+/// 四个根 tab 的容器：只有被访问过的 tab 才真正构建并常驻。
+///
+/// 冷启动时只建推荐页，其余 tab 等第一次切过去再建，省掉三棵子树和它们各自的请求。
+class _LazyTabStack extends StatefulWidget {
+  const _LazyTabStack({required this.index, required this.builders});
+
+  final int index;
+  final List<WidgetBuilder> builders;
+
+  @override
+  State<_LazyTabStack> createState() => _LazyTabStackState();
+}
+
+class _LazyTabStackState extends State<_LazyTabStack> {
+  late final Set<int> _visited = {widget.index};
+
+  @override
+  void didUpdateWidget(covariant _LazyTabStack oldWidget) {
+    super.didUpdateWidget(oldWidget);
+    _visited.add(widget.index);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return IndexedStack(
+      index: widget.index,
+      children: [
+        for (var index = 0; index < widget.builders.length; index++)
+          TickerMode(
+            enabled: widget.index == index,
+            child: _visited.contains(index)
+                ? widget.builders[index](context)
+                : const SizedBox.shrink(),
+          ),
+      ],
+    );
+  }
+}