refactor(app): flashcards keep the order they came in, without Reorder

Studying breaks the order of due cards anyway; what follows the list
order is new cards and cramming, and that order comes from creating and
importing, with sub-studysets putting lessons in order. No one drags a
card among a thousand (decided 2026-10-08).

Gone: Reorder and the grip on flashcards, CardListEventMoved,
CardRepository.move and the gateway's updatePosition, with their tests.
Each card keeps its stored position, so the order stays the same on
every device. liftWhileDragged is Material's plain lift again, for the
lists that keep Reorder.

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_de.arb

@@ -277,7 +277,6 @@
   "cardTextImported": "{count, plural, =1{1 Karteikarte importiert.} other{{count} Karteikarten importiert.}}",
   "studysetReorder": "Umsortieren",
   "studysetReorderPaths": "Pfade umsortieren",
-  "studysetReorderCards": "Karteikarten umsortieren",
   "studysetReorderDone": "Fertig",
   "studysetOpenTopics": "Themen anzeigen",
   "studysetTopics": "Themen",