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/anki/data/anki_importer_test.dart

@@ -176,6 +176,23 @@ void main() {
     expect(namesOf(cloze), isEmpty);
   });
 
+  test('a studyset made for an import that failed goes if nothing came '
+      'into it, and stays if something did', () async {
+    final empty = await importer.newStudysetFor(package, fallbackTitle: 'x');
+    await importer.discardIfEmpty(empty);
+    expect(
+      (await studysets.all()).map((studyset) => studyset.id),
+      isNot(contains(empty)),
+    );
+
+    await import();
+    await importer.discardIfEmpty(japanese);
+    expect(
+      (await studysets.all()).map((studyset) => studyset.id),
+      contains(japanese),
+    );
+  });
+
   test('makes a studyset of each deck below the top one', () async {
     await import(mode: AnkiImportMode.subStudysets);