feat(downloads): support up to 3 concurrent downloads

The Dart queue loop was already built around an activeDownloads map and
per-item progress tracking but was gated to one download at a time. Add
a concurrentDownloads setting (1-3, default 1) exposed in Download
Settings > Performance, and let the scheduler keep up to that many
items in flight. The experimental Android native worker is still
strictly sequential, so it is skipped in favor of the Dart queue while
concurrency is enabled.

lib/screens/settings/download_settings_page.dart

@@ -195,6 +195,20 @@ class _DownloadSettingsPageState extends ConsumerState<DownloadSettingsPage> {
                       settings.downloadNetworkMode,
                     ),
                   ),
+                  SettingsItem(
+                    icon: Icons.dynamic_feed_outlined,
+                    title: context.l10n.settingsConcurrentDownloads,
+                    subtitle: settings.concurrentDownloads <= 1
+                        ? context.l10n.concurrentDownloadsOne
+                        : context.l10n.concurrentDownloadsCount(
+                            settings.concurrentDownloads,
+                          ),
+                    onTap: () => _showConcurrentDownloadsPicker(
+                      context,
+                      ref,
+                      settings.concurrentDownloads,
+                    ),
+                  ),
                   if (Platform.isAndroid)
                     SettingsSwitchItem(
                       icon: Icons.downloading_outlined,
@@ -598,6 +612,73 @@ class _DownloadSettingsPageState extends ConsumerState<DownloadSettingsPage> {
     );
   }
 
+  void _showConcurrentDownloadsPicker(
+    BuildContext context,
+    WidgetRef ref,
+    int current,
+  ) {
+    final colorScheme = Theme.of(context).colorScheme;
+    showModalBottomSheet<void>(
+      context: context,
+      useRootNavigator: true,
+      backgroundColor: colorScheme.surfaceContainerHigh,
+      shape: const RoundedRectangleBorder(
+        borderRadius: BorderRadius.vertical(top: Radius.circular(28)),
+      ),
+      builder: (context) => SafeArea(
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          crossAxisAlignment: CrossAxisAlignment.start,
+          children: [
+            Padding(
+              padding: const EdgeInsets.fromLTRB(24, 24, 24, 8),
+              child: Text(
+                context.l10n.settingsConcurrentDownloads,
+                style: Theme.of(
+                  context,
+                ).textTheme.titleLarge?.copyWith(fontWeight: FontWeight.bold),
+              ),
+            ),
+            Padding(
+              padding: const EdgeInsets.fromLTRB(24, 0, 24, 16),
+              child: Text(
+                context.l10n.settingsConcurrentDownloadsSubtitle,
+                style: Theme.of(context).textTheme.bodyMedium?.copyWith(
+                  color: colorScheme.onSurfaceVariant,
+                ),
+              ),
+            ),
+            for (final count in const [1, 2, 3])
+              ListTile(
+                leading: Icon(
+                  count == 1
+                      ? Icons.looks_one_outlined
+                      : count == 2
+                      ? Icons.looks_two_outlined
+                      : Icons.looks_3_outlined,
+                ),
+                title: Text(
+                  count == 1
+                      ? context.l10n.concurrentDownloadsOne
+                      : context.l10n.concurrentDownloadsCount(count),
+                ),
+                trailing: current == count
+                    ? Icon(Icons.check, color: colorScheme.primary)
+                    : null,
+                onTap: () {
+                  ref
+                      .read(settingsProvider.notifier)
+                      .setConcurrentDownloads(count);
+                  Navigator.pop(context);
+                },
+              ),
+            const SizedBox(height: 16),
+          ],
+        ),
+      ),
+    );
+  }
+
   void _showSongLinkRegionPicker(
     BuildContext context,
     WidgetRef ref,