feat(ux): unify added-to-queue snackbars and add fetch retry

Every single-track enqueue now shows the shared added-to-queue
snackbar with a View action into Library (six call sites through one
helper). URL fetch failures in Home and via share intent gain a Retry
action; unrecognized-URL errors deliberately keep none since retrying
is deterministic.

lib/widgets/track_detail_actions.dart

@@ -8,6 +8,7 @@ import 'package:spotiflac_android/providers/library_collections_provider.dart';
 import 'package:spotiflac_android/providers/local_library_provider.dart';
 import 'package:spotiflac_android/providers/settings_provider.dart';
 import 'package:spotiflac_android/widgets/download_service_picker.dart';
+import 'package:spotiflac_android/widgets/view_queue_snackbar_action.dart';
 
 /// Shared single-track "add to queue" flow for detail screens: shows the
 /// quality/service picker when the user opted into it, otherwise resolves
@@ -24,10 +25,7 @@ void downloadSingleTrack(
   final settings = ref.read(settingsProvider);
 
   void notifyQueued() {
-    if (!context.mounted) return;
-    ScaffoldMessenger.of(context).showSnackBar(
-      SnackBar(content: Text(context.l10n.snackbarAddedToQueue(track.name))),
-    );
+    showAddedToQueueSnackBar(context, track.name);
   }
 
   if (settings.askQualityBeforeDownload || forceQualityPicker) {

lib/widgets/view_queue_snackbar_action.dart

@@ -10,3 +10,14 @@ SnackBarAction buildViewQueueSnackBarAction(BuildContext context) {
     },
   );
 }
+
+/// Shared "Added to queue" snackbar with a View action jumping to Library.
+void showAddedToQueueSnackBar(BuildContext context, String trackName) {
+  if (!context.mounted) return;
+  ScaffoldMessenger.of(context).showSnackBar(
+    SnackBar(
+      content: Text(context.l10n.snackbarAddedToQueue(trackName)),
+      action: buildViewQueueSnackBarAction(context),
+    ),
+  );
+}