feat: add Storage Access Framework (SAF) support for Android 10+

- Add SAF tree picker and persistent URI storage in settings
- Implement SAF file operations: exists, delete, stat, copy, create
- Update download pipeline to support SAF content URIs
- Add fallback to app-private storage when SAF write fails
- Support SAF in library scan with DocumentFile traversal
- Add history item repair for missing SAF URIs
- Create file_access.dart utilities for abstracted file operations
- Update Tidal/Qobuz/Amazon/Extensions for SAF-aware output
- Add runPostProcessingV2 API for SAF content URIs
- Update screens (album, artist, queue, track) for SAF awareness

Resolves Android 10+ scoped storage permission issues

lib/utils/file_access.dart

@@ -0,0 +1,66 @@
+import 'dart:io';
+
+import 'package:open_filex/open_filex.dart';
+import 'package:spotiflac_android/services/platform_bridge.dart';
+import 'package:spotiflac_android/utils/mime_utils.dart';
+
+class FileAccessStat {
+  final int? size;
+  final DateTime? modified;
+
+  const FileAccessStat({this.size, this.modified});
+}
+
+bool isContentUri(String? path) {
+  return path != null && path.startsWith('content://');
+}
+
+Future<bool> fileExists(String? path) async {
+  if (path == null || path.isEmpty) return false;
+  if (isContentUri(path)) {
+    return PlatformBridge.safExists(path);
+  }
+  return File(path).exists();
+}
+
+Future<void> deleteFile(String? path) async {
+  if (path == null || path.isEmpty) return;
+  if (isContentUri(path)) {
+    await PlatformBridge.safDelete(path);
+    return;
+  }
+  try {
+    await File(path).delete();
+  } catch (_) {}
+}
+
+Future<FileAccessStat?> fileStat(String? path) async {
+  if (path == null || path.isEmpty) return null;
+  if (isContentUri(path)) {
+    final stat = await PlatformBridge.safStat(path);
+    final exists = stat['exists'] as bool? ?? true;
+    if (!exists) return null;
+    return FileAccessStat(
+      size: stat['size'] as int?,
+      modified: stat['modified'] != null
+          ? DateTime.fromMillisecondsSinceEpoch(stat['modified'] as int)
+          : null,
+    );
+  }
+
+  final stat = await FileStat.stat(path);
+  if (stat.type == FileSystemEntityType.notFound) return null;
+  return FileAccessStat(size: stat.size, modified: stat.modified);
+}
+
+Future<void> openFile(String path) async {
+  if (isContentUri(path)) {
+    await PlatformBridge.openContentUri(path, mimeType: '');
+    return;
+  }
+  final mimeType = audioMimeTypeForPath(path);
+  final result = await OpenFilex.open(path, type: mimeType);
+  if (result.type != ResultType.done) {
+    throw Exception(result.message);
+  }
+}