revert(ui): drop hero cover flights from home/search to detail screens

The list-to-header cover flights read as a gimmick and add per-frame
cost during navigation. Removed the heroTag plumbing end to end
(recents, search rows, discography cards, Album/Artist/Downloaded
headers, motion banners). Kept: the mini player expand hero, the
library-to-track-metadata hero, real loading headers, and the
loading crossfade.

lib/widgets/album_detail_header.dart

@@ -23,7 +23,6 @@ class AlbumDetailHeader extends StatelessWidget {
     this.appBarTitle,
     this.leading,
     this.backgroundColor,
-    this.heroTag,
   });
 
   final String title;
@@ -62,10 +61,6 @@ 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;
@@ -154,7 +149,7 @@ class AlbumDetailHeader extends StatelessWidget {
                               final coverSize = (constraints.maxWidth * 0.5)
                                   .clamp(150.0, 210.0)
                                   .toDouble();
-                              final cover = Container(
+                              return Container(
                                 width: coverSize,
                                 height: coverSize,
                                 decoration: BoxDecoration(
@@ -174,9 +169,6 @@ class AlbumDetailHeader extends StatelessWidget {
                                   child: coverBuilder!(context, coverSize),
                                 ),
                               );
-                              return heroTag != null
-                                  ? Hero(tag: heroTag!, child: cover)
-                                  : cover;
                             },
                           ),
                           const SizedBox(height: 20),