feat(app): undo every step of a study session, deletions held until it ends

The header's Undo takes back ratings, burials and deletions, last first,
across the session's rounds and from the end of a round, and shows the
card again. A card deleted during a session is held (DelayedDeletions.hold)
until the session ends, then goes after the usual time to undo, so the
message offering Undo still works; if the app is killed first, the card
stays. Undo on a burial's or deletion's message takes back that step alone.

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_en.arb

@@ -537,7 +537,7 @@
   "studySettingsTagFilterPausedForBacklog": "This session is clearing your review backlog across every tag, so these tags aren't narrowing it. They apply again to your next session.",
   "studyBuryCard": "Bury until tomorrow",
   "studyCardBuried": "Card buried until tomorrow",
-  "studyUndoLastRating": "Undo last rating",
+  "studyUndo": "Undo",
   "studyLockScreen": "Lock screen",
   "studyUnlock": "Unlock",
   "studyTapToUnlock": "Tap to unlock",