feat(app): study with faces on the ratings, centred cards and the session's figures

The study session, reworked:

- Ratings are coloured tiles with a face each (frown, meh, smile,
  laugh), when the card comes back below them with a clock ("in 1
  min", "in 3 days"), always shown, so a card can be rated without
  revealing it; at most 540 dp wide, centred.
- Cards are centred in a section card: the question large, then a line
  with 24 dp on either side and the answer a size smaller, or "Show
  answer" 40 dp below; a tap anywhere on the card shows or hides the
  answer. Read-only rich text takes a text style and an alignment.
- Words show their reading small above the expression once answered,
  and several meanings numbered.
- Choice, matching and map cards: Check below the card, the ratings in
  its place once checked; options marked before their text; matching
  lists the categories first, then what is left to place; the map
  fills the card with one caption below it.
- Header and progress on a band of their own; title start-aligned,
  the Pomodoro timer only to watch, the back-first toggle in every
  state; leaving a round under way asks first.
- Cramming goes round the round; back is there with two cards or more,
  skipping always.
- The end of a round: the cards done of those due, accuracy (OK counts
  half) and time, over the whole session.

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

@@ -484,6 +484,21 @@
   "studyBackFirstOff": "Show the back of cards first",
   "studyShowAnswer": "Show answer",
   "studyCheck": "Check",
+  "studySelectAll": "Select all that apply",
+  "studySelectOne": "Select the correct answer",
+  "studyMatchingInstructions": "DRAG AND DROP ONTO CARDS ABOVE",
+  "studyMatchingAllPlaced": "All placed",
+  "studyMatchingDropHere": "Drop here",
+  "studyMapTapPrompt": "Tap a region to answer",
+  "studyMapSelected": "Selected: {label}",
+  "@studyMapSelected": {"placeholders": {"label": {"type": "String"}}},
+  "studyCorrect": "Correct",
+  "studyCorrectLabel": "Correct — {label}",
+  "@studyCorrectLabel": {"placeholders": {"label": {"type": "String"}}},
+  "studyMapAnswer": "Answer: {answer}",
+  "@studyMapAnswer": {"placeholders": {"answer": {"type": "String"}}},
+  "studyMapPickedAnswer": "You picked {picked} · Answer: {answer}",
+  "@studyMapPickedAnswer": {"placeholders": {"picked": {"type": "String"}, "answer": {"type": "String"}}},
   "studyRatingBad": "Bad",
   "studyRatingOk": "OK",
   "studyRatingGood": "Good",
@@ -493,13 +508,40 @@
   "studyNextDue": "The next card is due in {interval}.",
   "@studyNextDue": {"placeholders": {"interval": {"type": "String"}}},
   "studyDone": "Done",
-  "studyRoundDone": "Round complete",
+  "studySessionComplete": "Session complete",
+  "studyProgressReviewed": "{reviewed} / {total} reviewed",
+  "@studyProgressReviewed": {"placeholders": {"reviewed": {"type": "int"}, "total": {"type": "int"}}},
+  "studyReviewedCount": "{reviewed} reviewed",
+  "@studyReviewedCount": {"placeholders": {"reviewed": {"type": "int"}}},
+  "studyStatCards": "Cards",
+  "studyStatAccuracy": "Accuracy",
+  "studyStatTime": "Time",
+  "studyDurationSec": "{sec} sec",
+  "@studyDurationSec": {"placeholders": {"sec": {"type": "int"}}},
+  "studyDurationMin": "{min} min",
+  "@studyDurationMin": {"placeholders": {"min": {"type": "int"}}},
+  "studyDurationMinSec": "{min} min {sec} sec",
+  "@studyDurationMinSec": {"placeholders": {"min": {"type": "int"}, "sec": {"type": "int"}}},
+  "studyLeaveTitle": "Leave this study session?",
+  "studyLeaveRated": "{count, plural, =1{Your ratings so far are saved, but the last card in this round will be discarded if you leave now.} other{Your ratings so far are saved, but the {count} cards still left in this round will be discarded if you leave now.}}",
+  "@studyLeaveRated": {"placeholders": {"count": {"type": "int"}}},
+  "studyLeaveUnrated": "{count, plural, =1{You haven’t rated this card yet — leaving ends the session without recording any progress for this round.} other{You haven’t rated any of the {count} cards in this round yet — leaving ends the session without recording any progress for it.}}",
+  "@studyLeaveUnrated": {"placeholders": {"count": {"type": "int"}}},
+  "studyLeaveConfirm": "Leave",
+  "studyKeepStudying": "Keep studying",
+  "studyInMinutes": "{count, plural, =1{in 1 min} other{in {count} min}}",
+  "@studyInMinutes": {"placeholders": {"count": {"type": "int"}}},
+  "studyInHours": "{count, plural, =1{in 1 hour} other{in {count} hours}}",
+  "@studyInHours": {"placeholders": {"count": {"type": "int"}}},
+  "studyInDays": "{count, plural, =1{in 1 day} other{in {count} days}}",
+  "@studyInDays": {"placeholders": {"count": {"type": "int"}}},
+  "studyInMonths": "{count, plural, =1{in 1 month} other{in {count} months}}",
+  "@studyInMonths": {"placeholders": {"count": {"type": "int"}}},
+  "studyInYears": "{count, plural, =1{in 1 year} other{in {count} years}}",
+  "@studyInYears": {"placeholders": {"count": {"type": "int"}}},
   "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.",
+  "studySkipCard": "Skip card",
   "studyMinutes": "{count} min",
   "@studyMinutes": {"placeholders": {"count": {"type": "int"}}},
   "studyHours": "{count} h",