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/lib/l10n/app_localizations_en.dart

@@ -966,7 +966,7 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get cardTextImportDescription =>
-      'One card per line: its front, a separator, then its back. Each line is split at its first separator; lines without one are left out.';
+      'One card per line: its front, a separator, then its back. Each line is split at its first separator; lines without one are left out. A field in double quotes, as spreadsheets write them, may hold separators and line breaks.';
 
   @override
   String get cardTextImportHint => 'Paste the cards here';
@@ -978,7 +978,7 @@ class AppLocalizationsEn extends AppLocalizations {
   String get cardTextImportFileType => 'CSV or TSV files';
 
   @override
-  String get cardTextImportUnreadable => 'This file isn\'t text in UTF-8.';
+  String get cardTextImportUnreadable => 'This file doesn\'t hold text.';
 
   @override
   String get cardTextImportSeparator => 'Separator';
@@ -1542,6 +1542,15 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get tagsReadFailed => 'Couldn\'t read the tags.';
 
+  @override
+  String get notesReadFailed => 'Couldn\'t read the notes.';
+
+  @override
+  String get pathsReadFailed => 'Couldn\'t read the learning paths.';
+
+  @override
+  String get filesReadFailed => 'Couldn\'t read the files.';
+
   @override
   String get tagPickerTitle => 'Tags';