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/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}