fix(app): Reorder among older items, export and text import that hold up

- Reorder: rows from before an order existed all share one position, and
  one moved among them landed at their end; the run it goes into now
  gets positions of its own (placeAmong), for notes, files, paths,
  steps and sub-studysets. Rows whose deletion can still be undone count
  for nothing, as the list leaves them out, and a dropped row takes its
  place at once instead of snapping back until the order is read again.
- Export: two studysets inside each other (moved so on two devices
  offline) no longer make it run forever; links to files whose names
  hold #, % or brackets work; PDFs, images and sounds go into the zip
  without being compressed again; any error says the export failed.
- Text import: a real CSV reader (quoted fields over several lines, a
  back over several quoted fields, a stray quote spoiling only its own
  line), the separator guessed from the text (semicolons from German
  spreadsheets), and files in UTF-16 or Windows-1252 read as meant.
- A tab whose notes, paths or files couldn't be read says so rather than
  "No … yet"; an Anki import from the Library that fails leaves no empty
  studyset behind.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/test/feature/studyset/data/studyset_repository_test.dart

@@ -216,6 +216,31 @@ void main() {
 
       expect(await childrenTitles(japanese), ['Grammar', 'Kana', 'Kanji']);
     });
+
+    test('studysets from before they had an order, all at one position, '
+        'make room for one moved among them; those whose deletion waits '
+        'count for nothing', () async {
+      final japanese = await repository.create(titled('Japanese'));
+      for (final (id, title) in [
+        ('s1', 'A'),
+        ('s2', 'B'),
+        ('s3', 'C'),
+        ('s4', 'D'),
+      ]) {
+        gateway.receiveRemoteChange(
+          id,
+          encodeStudysetPayload(titled(title)),
+          parentId: japanese,
+        );
+      }
+
+      await repository.reorder('s4', toIndex: 1);
+      expect(await childrenTitles(japanese), ['A', 'D', 'B', 'C']);
+
+      // B's deletion waits: the list shows D, C, and A goes after C.
+      await repository.reorder('s1', toIndex: 2, hidden: {'s2'});
+      expect(await childrenTitles(japanese), ['D', 'B', 'C', 'A']);
+    });
   });
 
   test('blanks the content before deleting', () async {