fix(ios): use security-scoped bookmarks for download directory persistence

- Switch iOS bookmark creation from .minimalBookmark to .withSecurityScope
- Add .withSecurityScope option when resolving bookmarks
- Add downloadDirectoryBookmark field to AppSettings for persisting iOS bookmarks
- Resolve bookmark and startAccessingIosBookmark before queue processing
- Guarantee stopAccessingIosBookmark cleanup via try/finally
- Create bookmark on folder pick in both setup screen and download settings
- Clear bookmark when switching to SAF mode or iOS path normalization
- Fix stale bottom sheet context usage (ctx -> context) in download settings

ios/Runner/AppDelegate.swift

@@ -1021,7 +1021,7 @@ import Gobackend  // Import Go framework
         let url = URL(fileURLWithPath: path)
         do {
             let bookmarkData = try url.bookmarkData(
-                options: .minimalBookmark,
+                options: .withSecurityScope,
                 includingResourceValuesForKeys: nil,
                 relativeTo: nil
             )
@@ -1051,7 +1051,7 @@ import Gobackend  // Import Go framework
         do {
             url = try URL(
                 resolvingBookmarkData: bookmarkData,
-                options: [],
+                options: .withSecurityScope,
                 relativeTo: nil,
                 bookmarkDataIsStale: &isStale
             )
@@ -1086,7 +1086,7 @@ import Gobackend  // Import Go framework
         do {
             url = try URL(
                 resolvingBookmarkData: bookmarkData,
-                options: [],
+                options: .withSecurityScope,
                 relativeTo: nil,
                 bookmarkDataIsStale: &isStale
             )