fix(ui): hero continuity for motion (Apple Music) headers

Albums/artists with a motion banner have no square cover, so the Hero
had no destination: the open flight ended nowhere and there was no
return flight. The Hero now rides the full-bleed banner, flying the
still cover image as the shuttle so the video doesn't restart mid-air.

lib/screens/album_screen.dart

@@ -379,7 +379,7 @@ class _AlbumScreenState extends ConsumerState<AlbumScreen>
     final cacheWidth = coverCacheWidthForViewport(context);
     final headerBgUrl =
         _headerImageUrl ?? widget.headerImageUrl ?? widget.coverUrl;
-    final Widget headerBgImage = headerBgUrl != null
+    Widget headerBgImage() => headerBgUrl != null
         ? CachedNetworkImage(
             imageUrl: highResCoverUrl(headerBgUrl) ?? headerBgUrl,
             fit: BoxFit.cover,
@@ -397,15 +397,27 @@ class _AlbumScreenState extends ConsumerState<AlbumScreen>
             ),
           );
 
+    Widget background = hasMotion
+        ? MotionHeaderBanner(videoUrl: motionUrl, fallback: headerBgImage())
+        : headerBgImage();
+    // With a motion banner there is no square cover, so the Hero rides the
+    // full-bleed background instead; the shuttle flies the still image (a
+    // shuttle-built banner would restart the video mid-flight).
+    if (hasMotion && widget.heroTag != null) {
+      background = Hero(
+        tag: widget.heroTag!,
+        flightShuttleBuilder: (_, _, _, _, _) => headerBgImage(),
+        child: background,
+      );
+    }
+
     return AlbumDetailHeader(
       title: widget.albumName,
       expandedHeight: expandedHeight,
       showTitleInAppBar: showTitleInAppBar,
       backgroundColor: pageBackgroundColor,
       heroTag: widget.heroTag,
-      background: hasMotion
+      background: background,
-          ? MotionHeaderBanner(videoUrl: motionUrl, fallback: headerBgImage)
-          : headerBgImage,
       blurAndScrimBackground: showSquareCover,
       coverBuilder: showSquareCover
           ? (context, coverSize) => coverThumbUrl != null

lib/screens/artist_screen.dart

@@ -121,8 +121,18 @@ class _ArtistScreenState extends ConsumerState<ArtistScreen>
   int? _monthlyListeners;
   String? _error;
 
-  Widget _heroWrap(Widget child) =>
+  /// [shuttle], when given, is flown instead of re-instantiating [child]
-      widget.heroTag != null ? Hero(tag: widget.heroTag!, child: child) : child;
+  /// (needed for motion banners, which would restart their video mid-flight).
+  Widget _heroWrap(Widget child, {Widget Function()? shuttle}) =>
+      widget.heroTag != null
+      ? Hero(
+          tag: widget.heroTag!,
+          flightShuttleBuilder: shuttle == null
+              ? null
+              : (_, _, _, _, _) => shuttle(),
+          child: child,
+        )
+      : child;
 
   bool _showTitleInAppBar = false;
   final ScrollController _scrollController = ScrollController();
@@ -1170,34 +1180,42 @@ class _ArtistScreenState extends ConsumerState<ArtistScreen>
           fit: StackFit.expand,
           children: [
             if (hasMotionBanner)
-              MotionHeaderBanner(
+              Builder(
-                videoUrl: headerVideoUrl,
+                builder: (context) {
-                fallback: hasValidImage
+                  Widget fallbackImage() => hasValidImage
-                    ? CachedCoverImage(
+                      ? CachedCoverImage(
-                        imageUrl: imageUrl,
+                          imageUrl: imageUrl!,
-                        fit: BoxFit.cover,
+                          fit: BoxFit.cover,
-                        alignment: Alignment.topCenter,
+                          alignment: Alignment.topCenter,
-                        memCacheWidth: 800,
+                          memCacheWidth: 800,
-                        placeholder: (context, url) => Container(
+                          placeholder: (context, url) => Container(
-                          color: colorScheme.surfaceContainerHighest,
+                            color: colorScheme.surfaceContainerHighest,
-                        ),
+                          ),
-                        errorWidget: (context, url, error) => Container(
+                          errorWidget: (context, url, error) => Container(
+                            color: colorScheme.surfaceContainerHighest,
+                            child: Icon(
+                              Icons.person,
+                              size: 80,
+                              color: colorScheme.onSurfaceVariant,
+                            ),
+                          ),
+                        )
+                      : Container(
                           color: colorScheme.surfaceContainerHighest,
                           child: Icon(
                             Icons.person,
                             size: 80,
                             color: colorScheme.onSurfaceVariant,
                           ),
-                        ),
+                        );
-                      )
+                  return _heroWrap(
-                    : Container(
+                    MotionHeaderBanner(
-                        color: colorScheme.surfaceContainerHighest,
+                      videoUrl: headerVideoUrl!,
-                        child: Icon(
+                      fallback: fallbackImage(),
-                          Icons.person,
+                    ),
-                          size: 80,
+                    shuttle: fallbackImage,
-                          color: colorScheme.onSurfaceVariant,
+                  );
-                        ),
+                },
-                      ),
               )
             else if (hasValidImage)
               _heroWrap(