feat(ui): shared-element hero covers from search and recents to detail screens

Tab navigators get a HeroController (nested navigators have none, so
Heroes never flew inside tabs). Search result rows, recent-access items,
and artist discography cards tag their covers; Album/Artist/Downloaded
screens accept a heroTag and mount the matching Hero on the header
cover. Tags are scoped per list section to stay unique per page.

lib/widgets/album_detail_header.dart

@@ -23,6 +23,7 @@ class AlbumDetailHeader extends StatelessWidget {
     this.appBarTitle,
     this.leading,
     this.backgroundColor,
+    this.heroTag,
   });
 
   final String title;
@@ -61,6 +62,10 @@ class AlbumDetailHeader extends StatelessWidget {
 
   final Color? backgroundColor;
 
+  /// Shared-element tag: when set, the cover flies from the list item that
+  /// pushed this screen (which wraps its thumbnail in a matching [Hero]).
+  final Object? heroTag;
+
   /// Shrinks long titles so up to three lines fit the header.
   double _titleFontSize() {
     final length = title.trim().length;
@@ -149,7 +154,7 @@ class AlbumDetailHeader extends StatelessWidget {
                               final coverSize = (constraints.maxWidth * 0.5)
                                   .clamp(150.0, 210.0)
                                   .toDouble();
-                              return Container(
+                              final cover = Container(
                                 width: coverSize,
                                 height: coverSize,
                                 decoration: BoxDecoration(
@@ -169,6 +174,9 @@ class AlbumDetailHeader extends StatelessWidget {
                                   child: coverBuilder!(context, coverSize),
                                 ),
                               );
+                              return heroTag != null
+                                  ? Hero(tag: heroTag!, child: cover)
+                                  : cover;
                             },
                           ),
                           const SizedBox(height: 20),