feat(app): study sessions in rounds, by card type, cramming and reversed

A session studies a round of up to the settings' cards per round. In
spaced repetition a card leaves the round once it is a day or more away,
one rated lower comes back when due; cramming walks every card once,
round after round, its reviews moving no schedule, with going back and
on. Asking the back first (a toggle in the header, saved in the
settings) asks front/back and vocabulary cards their back, on their own
schedule. Each type studies its way: a word shows its reading and
meanings, a multiple-choice card is picked and checked, a matching card
sorted by dragging or tapping and checked; then the person rates (Bad,
OK, Good, Perfect, each with when the card comes back). Progress shows
as pills, or a tally bar while cramming; a round ends with how it went.
The session settings gain the mode, cards per round, new cards per day
and the order.

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

@@ -254,6 +254,55 @@
   "cardTileUntitled": "Untitled",
   "cardTileConflict": "Changed on two devices",
 
+  "studyClose": "Close session",
+  "studyBackFirstOn": "Showing the back of cards first: tap to revert",
+  "studyBackFirstOff": "Show the back of cards first",
+  "studyShowAnswer": "Show answer",
+  "studyCheck": "Check",
+  "studyRatingBad": "Bad",
+  "studyRatingOk": "OK",
+  "studyRatingGood": "Good",
+  "studyRatingPerfect": "Perfect",
+  "studyNothingTitle": "Nothing to study right now",
+  "studyNothingBody": "Come back later when more cards are due.",
+  "studyNextDue": "The next card is due in {interval}.",
+  "@studyNextDue": {"placeholders": {"interval": {"type": "String"}}},
+  "studyDone": "Done",
+  "studyRoundDone": "Round complete",
+  "studyContinue": "Continue learning",
+  "studyPrevious": "Previous card",
+  "studyNext": "Next card",
+  "studyReading": "Reading",
+  "studyMeanings": "Meanings",
+  "studyPlaceItems": "Drag each item onto its category, or tap an item and then a category.",
+  "studyMinutes": "{count} min",
+  "@studyMinutes": {"placeholders": {"count": {"type": "int"}}},
+  "studyHours": "{count} h",
+  "@studyHours": {"placeholders": {"count": {"type": "int"}}},
+  "studyDays": "{count, plural, =1{1 day} other{{count} days}}",
+  "@studyDays": {"placeholders": {"count": {"type": "int"}}},
+  "studyMonths": "{count, plural, =1{1 month} other{{count} months}}",
+  "@studyMonths": {"placeholders": {"count": {"type": "int"}}},
+  "studyYears": "{count, plural, =1{1 year} other{{count} years}}",
+  "@studyYears": {"placeholders": {"count": {"type": "int"}}},
+  "studyFilteredBy": "{count, plural, =1{Narrowed to 1 tag} other{Narrowed to {count} tags}}",
+  "@studyFilteredBy": {"placeholders": {"count": {"type": "int"}}},
+  "studySettingsCardsPerRound": "Cards per round",
+  "studySettingsNewPerDay": "New cards per day",
+  "studySettingsNewPerDayHint": "How many never-studied cards each studyset starts a day. Due reviews are never limited.",
+  "studySettingsUnlimited": "Unlimited",
+  "studySettingsSortBy": "Sort by",
+  "studySettingsLearnMode": "Learn mode",
+  "studyModeSpaced": "Spaced repetition",
+  "studyModeSpacedHint": "Due cards on their schedules; ratings move them.",
+  "studyModeCram": "Cram",
+  "studyModeCramHint": "Walk every card without touching the schedules.",
+  "studySortOldestFirst": "Oldest first",
+  "studySortRandom": "Random",
+  "studySortNewestFirst": "Newest first",
+  "studyFewer": "Fewer",
+  "studyMore": "More",
+
   "commonCancel": "Cancel",
   "commonClose": "Close",
   "commonUndo": "Undo",