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_localizations_de.dart

@@ -1527,6 +1527,47 @@ class AppLocalizationsDe extends AppLocalizations {
   @override
   String get studyCheck => 'Prüfen';
 
+  @override
+  String get studySelectAll => 'Wähle alle zutreffenden aus';
+
+  @override
+  String get studySelectOne => 'Wähle die richtige Antwort';
+
+  @override
+  String get studyMatchingInstructions => 'AUF DIE KARTEN OBEN ZIEHEN';
+
+  @override
+  String get studyMatchingAllPlaced => 'Alle platziert';
+
+  @override
+  String get studyMatchingDropHere => 'Hier ablegen';
+
+  @override
+  String get studyMapTapPrompt => 'Tippe auf eine Region, um zu antworten';
+
+  @override
+  String studyMapSelected(String label) {
+    return 'Ausgewählt: $label';
+  }
+
+  @override
+  String get studyCorrect => 'Richtig';
+
+  @override
+  String studyCorrectLabel(String label) {
+    return 'Richtig — $label';
+  }
+
+  @override
+  String studyMapAnswer(String answer) {
+    return 'Antwort: $answer';
+  }
+
+  @override
+  String studyMapPickedAnswer(String picked, String answer) {
+    return 'Du hast $picked gewählt · Antwort: $answer';
+  }
+
   @override
   String get studyRatingBad => 'Schlecht';
 
@@ -1555,7 +1596,129 @@ class AppLocalizationsDe extends AppLocalizations {
   String get studyDone => 'Fertig';
 
   @override
-  String get studyRoundDone => 'Runde abgeschlossen';
+  String get studySessionComplete => 'Sitzung abgeschlossen';
+
+  @override
+  String studyProgressReviewed(int reviewed, int total) {
+    return '$reviewed / $total wiederholt';
+  }
+
+  @override
+  String studyReviewedCount(int reviewed) {
+    return '$reviewed wiederholt';
+  }
+
+  @override
+  String get studyStatCards => 'Karten';
+
+  @override
+  String get studyStatAccuracy => 'Genauigkeit';
+
+  @override
+  String get studyStatTime => 'Zeit';
+
+  @override
+  String studyDurationSec(int sec) {
+    return '$sec Sek.';
+  }
+
+  @override
+  String studyDurationMin(int min) {
+    return '$min Min.';
+  }
+
+  @override
+  String studyDurationMinSec(int min, int sec) {
+    return '$min Min. $sec Sek.';
+  }
+
+  @override
+  String get studyLeaveTitle => 'Diese Lernsitzung verlassen?';
+
+  @override
+  String studyLeaveRated(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other:
+          'Deine bisherigen Bewertungen sind gespeichert, aber die $count noch verbleibenden Karten in dieser Runde werden verworfen, wenn du jetzt gehst.',
+      one: 'Deine bisherigen Bewertungen sind gespeichert, aber die letzte Karte in dieser Runde wird verworfen, wenn du jetzt gehst.',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String studyLeaveUnrated(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other:
+          'Du hast noch keine der $count Karten in dieser Runde bewertet — beim Verlassen endet die Sitzung, ohne Fortschritt dafür zu speichern.',
+      one: 'Du hast diese Karte noch nicht bewertet — beim Verlassen endet die Sitzung, ohne Fortschritt für diese Runde zu speichern.',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String get studyLeaveConfirm => 'Verlassen';
+
+  @override
+  String get studyKeepStudying => 'Weiter lernen';
+
+  @override
+  String studyInMinutes(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: 'in $count Min.',
+      one: 'in 1 Min.',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String studyInHours(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: 'in $count Stunden',
+      one: 'in 1 Stunde',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String studyInDays(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: 'in $count Tagen',
+      one: 'in 1 Tag',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String studyInMonths(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: 'in $count Monaten',
+      one: 'in 1 Monat',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String studyInYears(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: 'in $count Jahren',
+      one: 'in 1 Jahr',
+    );
+    return '$_temp0';
+  }
 
   @override
   String get studyContinue => 'Weiterlernen';
@@ -1564,17 +1727,7 @@ class AppLocalizationsDe extends AppLocalizations {
   String get studyPrevious => 'Vorherige Karte';
 
   @override
-  String get studyNext => 'Nächste Karte';
-
-  @override
-  String get studyReading => 'Lesung';
-
-  @override
-  String get studyMeanings => 'Bedeutungen';
-
-  @override
-  String get studyPlaceItems =>
-      'Ziehe jedes Element auf seine Kategorie oder tippe auf ein Element und dann auf eine Kategorie.';
+  String get studySkipCard => 'Karte überspringen';
 
   @override
   String studyMinutes(int count) {