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_en.arb

@@ -290,11 +290,11 @@
   "studysetImportCards": "Flashcards",
   "studysetImportCardsHint": "Pasted, or from a CSV or TSV file",
   "cardTextImportTitle": "Import flashcards",
-  "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.",
+  "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. A field in double quotes, as spreadsheets write them, may hold separators and line breaks.",
   "cardTextImportHint": "Paste the cards here",
   "cardTextImportLoadFile": "Load a CSV or TSV file",
   "cardTextImportFileType": "CSV or TSV files",
-  "cardTextImportUnreadable": "This file isn't text in UTF-8.",
+  "cardTextImportUnreadable": "This file doesn't hold text.",
   "cardTextImportSeparator": "Separator",
   "cardTextSeparatorTab": "Tab",
   "cardTextSeparatorComma": "Comma",
@@ -475,6 +475,9 @@
   "syncMismatchMore": "More",
 
   "tagsReadFailed": "Couldn't read the tags.",
+  "notesReadFailed": "Couldn't read the notes.",
+  "pathsReadFailed": "Couldn't read the learning paths.",
+  "filesReadFailed": "Couldn't read the files.",
   "tagPickerTitle": "Tags",
   "tagPickerManage": "Manage tags",
   "tagPickerSearchLabel": "Search or create a tag",