refactor(screens): share album-screen scaffolding and track flows

- SelectionModeMixin + CollapsingHeaderScrollMixin extracted;
  local/downloaded album screens now share AlbumTrackTile,
  AlbumScaffoldBody, DestructiveSelectionButton, HeaderMetaRow,
  and confirmAndDeleteTracks
- track_detail_actions.dart: shared downloadSingleTrack,
  queueTracksSkippingDownloaded, download-all confirm, queued
  snackbar, release-date formatter, list footer, love-all
- TtlCache<T> replaces the copied 10-minute static caches;
  album fetch branches share _applyAlbumMetadata
- playlist error card now uses ErrorCard; formatMegabytes shared
  by queue tab and update dialog

lib/utils/confirm_and_delete_tracks.dart

@@ -0,0 +1,54 @@
+import 'package:flutter/material.dart';
+import 'package:spotiflac_android/l10n/l10n.dart';
+
+/// Shows a delete-confirmation dialog, deletes the given [ids] one by one via
+/// [deleteItem], then shows a "deleted N tracks" snackbar.
+/// [onExitSelectionMode] runs right after the delete loop, matching the
+/// screens' original ordering.
+///
+/// Returns the number of items [deleteItem] reported deleted, or null if the
+/// user cancelled the dialog.
+Future<int?> confirmAndDeleteTracks({
+  required BuildContext context,
+  required List<String> ids,
+  required Future<bool> Function(String id) deleteItem,
+  required VoidCallback onExitSelectionMode,
+}) async {
+  final confirmed = await showDialog<bool>(
+    context: context,
+    builder: (ctx) => AlertDialog(
+      title: Text(context.l10n.downloadedAlbumDeleteSelected),
+      content: Text(context.l10n.downloadedAlbumDeleteMessage(ids.length)),
+      actions: [
+        TextButton(
+          onPressed: () => Navigator.pop(ctx, false),
+          child: Text(context.l10n.dialogCancel),
+        ),
+        FilledButton(
+          onPressed: () => Navigator.pop(ctx, true),
+          style: FilledButton.styleFrom(
+            backgroundColor: Theme.of(context).colorScheme.error,
+          ),
+          child: Text(context.l10n.dialogDelete),
+        ),
+      ],
+    ),
+  );
+
+  if (confirmed != true || !context.mounted) return null;
+
+  var deletedCount = 0;
+  for (final id in ids) {
+    if (await deleteItem(id)) deletedCount++;
+  }
+
+  onExitSelectionMode();
+
+  if (context.mounted) {
+    ScaffoldMessenger.of(context).showSnackBar(
+      SnackBar(content: Text(context.l10n.snackbarDeletedTracks(deletedCount))),
+    );
+  }
+
+  return deletedCount;
+}

lib/utils/string_utils.dart

@@ -55,6 +55,11 @@ String? normalizeRemoteHttpUrl(String? value) {
   return null;
 }
 
+/// Byte count expressed as a plain megabyte number with 1 decimal, e.g. "3.4".
+String formatMegabytes(num bytes) {
+  return (bytes / (1024 * 1024)).toStringAsFixed(1);
+}
+
 /// Human-readable byte size: "512 B", "3.4 KB", "12.0 MB", "1.25 GB".
 String formatBytes(int bytes) {
   if (bytes < 1024) return '$bytes B';

lib/utils/ttl_cache.dart

@@ -0,0 +1,27 @@
+/// Simple in-memory cache where each entry expires after [ttl].
+class TtlCache<T> {
+  final Duration ttl;
+  final Map<String, _TtlEntry<T>> _entries = {};
+
+  TtlCache(this.ttl);
+
+  T? get(String key) {
+    final entry = _entries[key];
+    if (entry == null) return null;
+    if (DateTime.now().isAfter(entry.expiresAt)) {
+      _entries.remove(key);
+      return null;
+    }
+    return entry.value;
+  }
+
+  void set(String key, T value) {
+    _entries[key] = _TtlEntry(value, DateTime.now().add(ttl));
+  }
+}
+
+class _TtlEntry<T> {
+  final T value;
+  final DateTime expiresAt;
+  _TtlEntry(this.value, this.expiresAt);
+}