feat(recommend): 上新板块改按后端语义码判断，顶部轮播改为无缝循环

Drama 新增 tagCode，推荐页只看 NEW_ARRIVAL 这个码决定上新横滑，
不再依赖可变的标签文案；分类网格高度改为复用卡片的统一常量。
头部轮播改用虚拟无限页码，首尾相接且能反向滑，自动播放不再倒滑回第一张。

lib/pages/recommend/widgets/home_hero_carousel.dart

@@ -46,15 +46,25 @@ class _HomeHeroCarouselState extends State<HomeHeroCarousel> {
   static const double _dotGap = 9;
   static const double _ctaHeight = 38;
 
-  final PageController _controller = PageController();
+  /// 无限轮播：PageView 用虚拟大页码，取模映射到真实 banner，首尾天然相接。
+  static const int _basePage = 1000;
+
+  late final PageController _controller;
   Timer? _timer;
+  int _page = 0;
   int _index = 0;
   bool _pressed = false;
   bool _tickerEnabled = true;
 
+  /// 起始页码取 banner 数量的大倍数，向左右滑都不会到头。
+  static int _initialPageOf(int bannerCount) =>
+      bannerCount > 1 ? bannerCount * _basePage : 0;
+
   @override
   void initState() {
     super.initState();
+    _page = _initialPageOf(widget.banners.length);
+    _controller = PageController(initialPage: _page);
     WidgetsBinding.instance.addPostFrameCallback((_) => _syncTimer());
   }
 
@@ -72,8 +82,11 @@ class _HomeHeroCarouselState extends State<HomeHeroCarousel> {
   void didUpdateWidget(covariant HomeHeroCarousel oldWidget) {
     super.didUpdateWidget(oldWidget);
     if (oldWidget.banners.length != widget.banners.length) {
-      if (_index >= widget.banners.length) _index = 0;
-      _syncTimer();
+      // 首屏 banner 是异步到的：列表从空变有之后要把页码挪到虚拟起点，
+      // 否则左滑第一下就到头了。
+      _page = _initialPageOf(widget.banners.length);
+      _index = 0;
+      _jumpToBasePage();
     }
     if (oldWidget.banners.length != widget.banners.length ||
         oldWidget.active != widget.active) {
@@ -91,7 +104,8 @@ class _HomeHeroCarouselState extends State<HomeHeroCarousel> {
   void _syncTimer() {
     _timer?.cancel();
     _timer = null;
-    final canAdvance = mounted &&
+    final canAdvance =
+        mounted &&
         widget.active &&
         _tickerEnabled &&
         !_pressed &&
@@ -102,12 +116,16 @@ class _HomeHeroCarouselState extends State<HomeHeroCarousel> {
 
   void _advance() {
     if (!_controller.hasClients || widget.banners.length < 2) return;
-    final next = (_index + 1) % widget.banners.length;
-    _controller.animateToPage(
-      next,
-      duration: _pageDuration,
-      curve: Curves.easeOutCubic,
-    );
+    // 虚拟页码无限，直接翻下一页即可，不用在末尾跳回第 0 页。
+    _controller.nextPage(duration: _pageDuration, curve: Curves.easeOutCubic);
+  }
+
+  /// 把 PageView 静默挪到虚拟起点：视觉上是同一张图，不会闪。
+  void _jumpToBasePage() {
+    WidgetsBinding.instance.addPostFrameCallback((_) {
+      if (!mounted || !_controller.hasClients) return;
+      if (_controller.page?.round() != _page) _controller.jumpToPage(_page);
+    });
   }
 
   @override
@@ -134,9 +152,17 @@ class _HomeHeroCarouselState extends State<HomeHeroCarousel> {
           children: [
             PageView.builder(
               controller: _controller,
-              itemCount: widget.banners.length,
-              onPageChanged: (value) => setState(() => _index = value),
-              itemBuilder: (context, index) => _bannerPage(context, index),
+              // 数量大于 1 时用无限页码做无缝循环，取模映射到真实 banner。
+              itemCount: widget.banners.length > 1
+                  ? null
+                  : widget.banners.length,
+              onPageChanged: (value) => setState(() {
+                _page = value;
+                final count = widget.banners.length;
+                _index = count == 0 ? 0 : value % count;
+              }),
+              itemBuilder: (context, page) =>
+                  _bannerPage(context, page % widget.banners.length),
             ),
             Positioned(left: 0, right: 0, bottom: 14, child: _dots(context)),
           ],
@@ -288,16 +314,18 @@ class _HeroCopy extends StatelessWidget {
             color: AppColors.textPrimary.withValues(alpha: 0.95),
             fontSize: 15,
             fontWeight: FontWeight.w400,
-            shadows: const [
-              Shadow(color: Color(0x80000000), blurRadius: 8),
-            ],
+            shadows: const [Shadow(color: Color(0x80000000), blurRadius: 8)],
           ),
         ),
         const SizedBox(height: 20),
         Row(
           mainAxisSize: MainAxisSize.min,
           children: [
-            _WatchButton(height: ctaHeight, label: l10n.watchNowAction, onTap: onWatch),
+            _WatchButton(
+              height: ctaHeight,
+              label: l10n.watchNowAction,
+              onTap: onWatch,
+            ),
             const SizedBox(width: 12),
             _FavoriteCircle(size: ctaHeight, drama: drama),
           ],