refactor(screens): migrate remaining screens to shared header and selection components

- album/playlist/library-folder/track-metadata adopt
  CollapsingHeaderScrollMixin; artist/library-folder adopt
  SelectionModeMixin
- HeaderCircleButton and HeaderMetaRow replace per-screen copies
  in album and playlist headers; dead _tallHeader flag removed
- SelectionBottomBar gains optional subtitle labels and backs the
  playlist selection bar in queue tab

lib/widgets/album_detail_header.dart

@@ -283,6 +283,45 @@ class AlbumPlayActions extends StatelessWidget {
   }
 }
 
+/// 48x48 translucent-white circle icon button used in album/playlist headers
+/// (add-to-playlist, love-all, ...).
+class HeaderCircleButton extends StatelessWidget {
+  const HeaderCircleButton({
+    super.key,
+    required this.icon,
+    required this.tooltip,
+    required this.onPressed,
+    this.iconColor = Colors.white,
+  });
+
+  final IconData icon;
+  final String tooltip;
+  final VoidCallback? onPressed;
+  final Color iconColor;
+
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      width: 48,
+      height: 48,
+      decoration: BoxDecoration(
+        shape: BoxShape.circle,
+        color: Colors.white.withValues(alpha: 0.15),
+        border: Border.all(
+          color: Colors.white.withValues(alpha: 0.3),
+          width: 1,
+        ),
+      ),
+      child: IconButton(
+        onPressed: onPressed,
+        icon: Icon(icon, size: 22, color: iconColor),
+        tooltip: tooltip,
+        padding: EdgeInsets.zero,
+      ),
+    );
+  }
+}
+
 /// "•"-joined row of white [HeaderMetaItem]s shown under the header subtitle
 /// (track count, duration, quality, ...).
 class HeaderMetaRow extends StatelessWidget {

lib/widgets/selection_bottom_bar.dart

@@ -13,6 +13,8 @@ class SelectionBottomBar extends StatelessWidget {
     required this.onToggleSelectAll,
     required this.bottomPadding,
     required this.children,
+    this.allSelectedLabel,
+    this.tapToSelectLabel,
   });
 
   final int selectedCount;
@@ -22,6 +24,12 @@ class SelectionBottomBar extends StatelessWidget {
   final double bottomPadding;
   final List<Widget> children;
 
+  /// Overrides the default "All tracks selected" subtitle.
+  final String? allSelectedLabel;
+
+  /// Overrides the default "Tap tracks to select" subtitle.
+  final String? tapToSelectLabel;
+
   @override
   Widget build(BuildContext context) {
     final colorScheme = Theme.of(context).colorScheme;
@@ -80,8 +88,10 @@ class SelectionBottomBar extends StatelessWidget {
                         ),
                         Text(
                           allSelected
-                              ? context.l10n.selectionAllSelected
-                              : context.l10n.downloadedAlbumTapToSelect,
+                              ? allSelectedLabel ??
+                                    context.l10n.selectionAllSelected
+                              : tapToSelectLabel ??
+                                    context.l10n.downloadedAlbumTapToSelect,
                           style: Theme.of(context).textTheme.bodySmall
                               ?.copyWith(color: colorScheme.onSurfaceVariant),
                         ),