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.
Abelardo Ramirez