feat(app): bury a card until tomorrow while studying

"Bury until tomorrow" in a card's menu sets it aside for the rest of the
local day: it leaves the round, and no session or count takes it until
the next day begins; its schedule stays as it was. Undo, offered for a
few seconds, brings it back into the round.

A burial is an entry of its own in the review log (mode "bury"), as a
fresh start is: no synced table changes, older app versions skip it as
a mode they don't know, and it never counts as studying.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_localizations_de.dart

@@ -1550,6 +1550,12 @@ class AppLocalizationsDe extends AppLocalizations {
   @override
   String get studyClose => 'Sitzung schließen';
 
+  @override
+  String get studyBuryCard => 'Bis morgen zurückstellen';
+
+  @override
+  String get studyCardBuried => 'Karte bis morgen zurückgestellt';
+
   @override
   String get studyUndoLastRating => 'Letzte Bewertung rückgängig machen';