fix(app): poster sheet close button — header row with explicit close (drag-down not obvious on long sheet)

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

apps/mobile/lib/pages/share_qr_dialog.dart

@@ -52,7 +52,8 @@ typedef ShareLauncher = Future<void> Function(String text);
 
 final shareLauncherProvider = Provider<ShareLauncher>(
   // share_plus 11：SharePlus.instance.share（旧 Share.share 已废弃）。
-  (ref) => (text) async => SharePlus.instance.share(
+  (ref) =>
+      (text) async => SharePlus.instance.share(
         ShareParams(text: text, subject: text.split('\n').first),
       ),
 );
@@ -332,6 +333,22 @@ class _PosterSheetState extends ConsumerState<_PosterSheet> {
           child: Column(
             mainAxisSize: MainAxisSize.min,
             children: [
+              // ── 头行：标题 + 显式关闭（海报较长时下拉手势不明显）──
+              Row(
+                children: [
+                  Expanded(
+                    child: Text(
+                      l10n.sharePosterMode,
+                      style: theme.textTheme.titleMedium,
+                    ),
+                  ),
+                  IconButton(
+                    tooltip: l10n.close,
+                    icon: const Icon(Icons.close),
+                    onPressed: () => Navigator.of(context).pop(),
+                  ),
+                ],
+              ),
               // ── 海报（整块 RepaintBoundary；下载即渲染此区域）──
               RepaintBoundary(
                 key: _posterKey,