feat(app): lock the study screen, and undo the last rating

- Lock screen (header): a light scrim takes every touch; a tap shows
  Unlock for three seconds. Time locked doesn't count to the session's,
  and a running Pomodoro timer pauses, going on once unlocked (also
  when the session is left locked).
- Undo last rating (header): takes back one rating at a time within the
  round, its review deleted from the log, the round as it was; the card
  shows again from its question. Starting a round anew, or deleting a
  card, clears what can be undone.
- The review log deletes entries of any kind made at a moment (resets
  and ratings alike).

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

@@ -487,6 +487,10 @@
   "cardTileConflict": "Changed on two devices",
 
   "studyClose": "Close session",
+  "studyUndoLastRating": "Undo last rating",
+  "studyLockScreen": "Lock screen",
+  "studyUnlock": "Unlock",
+  "studyTapToUnlock": "Tap to unlock",
   "studyLoadFailed": "Couldn't read this session.",
   "studyRatingFailed": "Couldn't keep your rating. Rate the card again.",
   "studyRetry": "Retry",