feat(app): 海报弹层内中/English 切换——二维码海报支持英文版

- 弹层顶部 SegmentTabs(light) 两档，默认跟 App 语言前缀；只影响海报内容
  与二维码 ?lang=，不动 App 语言设置
- 提示文案经 lookupAppLocalizations 按海报语言取（arb 中英双全，零硬编码）
- 标题/场地/地址复用相册多语言回退链按海报语言取
- QrImageView 补 semanticsLabel（读屏原本无名，兼作测试断言口）

Co-Authored-By: Claude Code <noreply@anthropic.com>

apps/mobile/lib/pages/share_qr_dialog.dart

@@ -27,6 +27,7 @@ import '../features/albums/album_format.dart';
 import '../features/albums/models.dart';
 import '../l10n/app_localizations.dart';
 import '../ui/components/photo_thumbnail.dart';
+import '../ui/components/segment_tabs.dart';
 import '../ui/tokens.dart';
 
 /// 外链打开注入点（生产=url_launcher；测试 override）。
@@ -325,6 +326,9 @@ class ShareQrPage extends ConsumerWidget {
 
 /// 二维码海报弹层：宣传图（banner→cover 兜底）+ 标题 + 时间 + 地址 + 二维码；
 /// 「保存海报图」= RepaintBoundary 3x 渲染 PNG → 相册（gal）。
+///
+/// 弹层内「中文/English」切换海报语言（默认跟 App 语言）：内容文案/相册
+/// 字段/二维码 `?lang=` 随切换，App 语言设置与弹层 chrome 不受影响。
 class _PosterSheet extends ConsumerStatefulWidget {
   const _PosterSheet({required this.album});
 
@@ -339,6 +343,12 @@ class _PosterSheetState extends ConsumerState<_PosterSheet> {
   String? _bannerUrl;
   bool _saving = false;
 
+  /// 海报语言（'zh'|'en'；打开时刻快照 App 语言前缀，切换只影响海报）。
+  late String _posterLang =
+      AppLocalizations.of(context).localeName.toLowerCase().startsWith('zh')
+          ? 'zh'
+          : 'en';
+
   @override
   void initState() {
     super.initState();
@@ -377,9 +387,10 @@ class _PosterSheetState extends ConsumerState<_PosterSheet> {
   Widget build(BuildContext context) {
     final l10n = AppLocalizations.of(context);
     final theme = Theme.of(context);
-    final localeName = l10n.localeName;
-    final url = shareAlbumUrl(widget.album, localeName);
-    final title = albumDisplayTitle(widget.album, localeName);
+    // 海报内容按海报语言渲染（弹层 chrome 仍跟 App 语言）。
+    final posterL10n = lookupAppLocalizations(Locale(_posterLang));
+    final url = shareAlbumUrl(widget.album, _posterLang);
+    final title = albumDisplayTitle(widget.album, _posterLang);
     // 海报日期：日期粒度（UI06 选择器已去时间；展示同口径）。
     String posterDate(DateTime utc) {
       final wall = tryToAlbumZone(utc, widget.album.timezone) ?? utc.toUtc();
@@ -391,9 +402,9 @@ class _PosterSheetState extends ConsumerState<_PosterSheet> {
         ? null
         : posterDate(widget.album.endAt!);
     final dateRange = end == null ? start : '$start ~ $end';
-    final venue = albumDisplayVenue(widget.album, localeName);
+    final venue = albumDisplayVenue(widget.album, _posterLang);
     String? addressForPoster(AlbumSummary album) {
-      for (final t in albumDisplayTranslations(album, localeName)) {
+      for (final t in albumDisplayTranslations(album, _posterLang)) {
         final a = t.address?.trim();
         if (a != null && a.isNotEmpty) return a;
       }
@@ -426,6 +437,18 @@ class _PosterSheetState extends ConsumerState<_PosterSheet> {
                   ),
                 ],
               ),
+              const SizedBox(height: kSpace8),
+              // ── 海报语言切换：内容/二维码随之，App 语言不动 ──
+              SegmentTabs<String>(
+                value: _posterLang,
+                onChanged: (lang) => setState(() => _posterLang = lang),
+                style: SegmentTabsStyle.light,
+                segments: [
+                  ('zh', l10n.localeChinese),
+                  ('en', l10n.localeEnglish),
+                ],
+              ),
+              const SizedBox(height: kSpace12),
               // ── 海报（整块 RepaintBoundary；下载即渲染此区域）──
               RepaintBoundary(
                 key: _posterKey,
@@ -520,12 +543,13 @@ class _PosterSheetState extends ConsumerState<_PosterSheet> {
                                 version: QrVersions.auto,
                                 size: 200,
                                 backgroundColor: Colors.white,
+                                semanticsLabel: url,
                               ),
                             ),
                             const SizedBox(height: kSpace12),
                             Center(
                               child: Text(
-                                l10n.shareScanHint,
+                                posterL10n.shareScanHint,
                                 style: theme.textTheme.bodySmall?.copyWith(
                                   color: const Color(0xFF69778E),
                                 ),