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_de.arb

@@ -443,6 +443,8 @@
   "cardTileConflict": "Auf zwei Geräten geändert",
 
   "studyClose": "Sitzung schließen",
+  "studyBuryCard": "Bis morgen zurückstellen",
+  "studyCardBuried": "Karte bis morgen zurückgestellt",
   "studyUndoLastRating": "Letzte Bewertung rückgängig machen",
   "studyLockScreen": "Bildschirm sperren",
   "studyUnlock": "Entsperren",