fix(ios): stop dropping the security-scoped download folder bookmark

Fixes #439, likely also #302.

Reported: on iOS, the download folder silently reverts to the default
SpotiFLAC folder after starting a download (and stays reverted after
closing and reopening the app).

_processQueue ran an iCloud/writable-path shape check against the
persisted downloadDirectory string and, if it looked like iCloud Drive
or failed the structural writable-path check, called
setDownloadDirectory() to reset it to the default folder. That call
passes no iosBookmark argument, so it also wipes the user's
security-scoped bookmark in the persisted settings - permanently, not
just for the current run.

This check ran unconditionally, even when a bookmark was already
present for that folder. A folder picked from Files can legitimately
have a persisted path that looks like iCloud Drive, or that fails this
generic structural check, while the bookmark itself still grants real
write access - the bookmark is the actual source of truth, and the
queue already has a separate, correct bookmark-resolution step right
after this one (StartAccessingIosBookmark, with its own proper
failure handling that fails the queued items with a clear message
instead of destroying the setting). The earlier path-shape check just
never deferred to it.

lib/utils/file_access.dart: extract the run/skip decision into
shouldValidateIosOutputDir(isIOS, isSafMode, outputDir,
downloadDirectoryBookmark) - skips whenever a bookmark is present -
and use it in download_queue_provider.dart in place of the inline
condition. Kept as a plain function taking isIOS as a parameter since
Platform.isIOS itself can't be exercised from a host test run.

test/file_access_ios_test.dart covers: runs for a plain app-folder
path, skips when a bookmark is present (the exact #439 case), skips
off iOS, skips in SAF mode, and skips when there's no output dir yet.

Verification: flutter analyze and flutter test (224 tests, all green,
including the 5 new ones) both clean.

lib/providers/download_queue_provider.dart

@@ -1470,8 +1470,15 @@ class DownloadQueueNotifier extends Notifier<DownloadQueueState> {
       await _initOutputDir();
     }
 
-    // iOS: Validate that outputDir is writable (not iCloud Drive which Go can't access)
+    // iOS: Validate that outputDir is writable (not iCloud Drive which Go
-    if (!isSafMode && Platform.isIOS && state.outputDir.isNotEmpty) {
+    // can't access), unless a bookmark makes this app-Documents path-shape
+    // check irrelevant (see shouldValidateIosOutputDir).
+    if (shouldValidateIosOutputDir(
+      isIOS: Platform.isIOS,
+      isSafMode: isSafMode,
+      outputDir: state.outputDir,
+      downloadDirectoryBookmark: settings.downloadDirectoryBookmark,
+    )) {
       final isICloudPath =
           state.outputDir.contains('Mobile Documents') ||
           state.outputDir.contains('CloudDocs') ||

lib/utils/file_access.dart

@@ -6,6 +6,30 @@ import 'package:spotiflac_android/services/music_player_service.dart';
 import 'package:spotiflac_android/services/platform_bridge.dart';
 import 'package:spotiflac_android/utils/mime_utils.dart';
 
+/// Whether the queue should run its iOS output-directory path-shape check
+/// (which can replace [outputDir] with the default Documents folder when the
+/// path looks like iCloud Drive or an invalid container path).
+///
+/// A security-scoped bookmark is the source of truth for a folder picked
+/// from Files: its persisted path may legitimately contain substrings that
+/// look like iCloud Drive, or fail the structural writable-path check, even
+/// though the bookmark itself grants real write access. Running the
+/// path-shape check anyway would reset - and via setDownloadDirectory,
+/// permanently drop - a perfectly valid bookmarked folder. When a bookmark
+/// is present, the queue's later bookmark-resolution step is authoritative
+/// instead.
+bool shouldValidateIosOutputDir({
+  required bool isIOS,
+  required bool isSafMode,
+  required String outputDir,
+  required String downloadDirectoryBookmark,
+}) {
+  return isIOS &&
+      !isSafMode &&
+      outputDir.isNotEmpty &&
+      downloadDirectoryBookmark.isEmpty;
+}
+
 /// Regular expression to detect iOS app container paths.
 /// Matches paths like /var/mobile/Containers/Data/Application/{UUID}
 /// or /private/var/mobile/Containers/Data/Application/{UUID}

test/file_access_ios_test.dart

@@ -0,0 +1,76 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:spotiflac_android/utils/file_access.dart';
+
+void main() {
+  group('shouldValidateIosOutputDir', () {
+    test('validates a non-empty app-folder path on iOS', () {
+      expect(
+        shouldValidateIosOutputDir(
+          isIOS: true,
+          isSafMode: false,
+          outputDir:
+              '/private/var/mobile/Containers/Data/Application/ABC/Documents/SpotiFLAC',
+          downloadDirectoryBookmark: '',
+        ),
+        isTrue,
+      );
+    });
+
+    test(
+      'skips validation when a security-scoped bookmark is present (#439)',
+      () {
+        // The persisted path may look like iCloud Drive or fail the
+        // structural writable-path check even though the bookmark grants
+        // real write access - the bookmark, not this path shape, is
+        // authoritative. Running the check anyway is what silently reset
+        // and dropped the user's chosen folder.
+        expect(
+          shouldValidateIosOutputDir(
+            isIOS: true,
+            isSafMode: false,
+            outputDir:
+                '/private/var/mobile/Library/Mobile Documents/com~apple~CloudDocs/Music',
+            downloadDirectoryBookmark: 'bookmark-bytes',
+          ),
+          isFalse,
+        );
+      },
+    );
+
+    test('skips validation off iOS', () {
+      expect(
+        shouldValidateIosOutputDir(
+          isIOS: false,
+          isSafMode: false,
+          outputDir: '/data/user/0/com.zarz.spotiflac/files/SpotiFLAC',
+          downloadDirectoryBookmark: '',
+        ),
+        isFalse,
+      );
+    });
+
+    test('skips validation in SAF mode', () {
+      expect(
+        shouldValidateIosOutputDir(
+          isIOS: true,
+          isSafMode: true,
+          outputDir: '/some/path',
+          downloadDirectoryBookmark: '',
+        ),
+        isFalse,
+      );
+    });
+
+    test('skips validation when there is no output directory yet', () {
+      expect(
+        shouldValidateIosOutputDir(
+          isIOS: true,
+          isSafMode: false,
+          outputDir: '',
+          downloadDirectoryBookmark: '',
+        ),
+        isFalse,
+      );
+    });
+  });
+}