fix(app): studyset tabs built as they scroll, and a Reorder mode for flashcards

A studyset of 1,200 flashcards froze the window for seconds on opening
(7.4 s in a debug test; a release build is faster but still grows with
every card), long enough for the desktop to offer to kill the app: the
tabs held each list as one child of a scrolling view, shrink-wrapped, so
every row was built and laid out at once, and again on every change.

The tabs are now CustomScrollViews holding each list's slivers, built as
they scroll into view (0.07 s for the same 1,200 cards). What a list says
in place of its rows sits in SliverCentered.

On the desktop, Flutter's default drag handle lay over each card's "…",
so a press there began a drag rather than opening the options. Cards now
move in a Reorder mode, as paths do: Reorder beside the tabs (with two
cards or more, all showing), a grip in place of each card's options,
Done to end it. ReorderToggle and liftWhileDragged are shared with the
paths.

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

@@ -277,6 +277,7 @@
   "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",
@@ -284,8 +285,6 @@
   "studysetNewSubStudyset": "Neues Unter-Lernset",
   "studysetSubStudysetsEmpty": "Noch keine Unter-Lernsets.",
   "studysetSubStudysetsEmptyHint": "Teile dieses Lernset in Meilensteine oder Lektionen auf.",
-  "studysetReorder": "Umsortieren",
-  "studysetReorderDone": "Fertig",
   "studysetDragToReorder": "Zum Umsortieren ziehen",
   "studysetMoveTitle": "Lernset verschieben nach",
   "studysetMoveTopLevel": "Oberste Ebene",