feat(app): give cards hints and an explanation

A card of any type can carry up to six hints and an explanation, both
rich text. They are written in the composer, added from its menu (or
Ctrl+Alt+H / Ctrl+Alt+E), each removed on its own; blank ones aren't
kept. While studying, "Hints" under the question opens them one at a
time, and "Explanation" comes with the answer.

Card schema 5 holds them (`hints`, `explanation`), and with them what a
card needs next: a report that something on it is wrong (`report`:
category, note, when) and a word's senses with their grammar codes
(`senses`; a word's meanings then come from them, and its first sense
says whether it is usually written in kana). Each merges as a field of
its own.

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_en.dart

@@ -1216,6 +1216,52 @@ class AppLocalizationsEn extends AppLocalizations {
     return '+$_temp0';
   }
 
+  @override
+  String get cardHints => 'Hints';
+
+  @override
+  String cardHint(int number) {
+    return 'Hint $number';
+  }
+
+  @override
+  String get cardAddHint => 'Add hint';
+
+  @override
+  String get cardRemoveHint => 'Remove hint';
+
+  @override
+  String get cardExplanation => 'Explanation';
+
+  @override
+  String get cardAddExplanation => 'Add explanation';
+
+  @override
+  String get cardRemoveExplanation => 'Remove explanation';
+
+  @override
+  String cardHintsSheetTitle(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: 'Hints  $count',
+      one: 'Hint',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String get cardPreviousHint => 'Previous hint';
+
+  @override
+  String get cardNextHint => 'Next hint';
+
+  @override
+  String get cardReport => 'Report';
+
+  @override
+  String get cardSenses => 'Senses';
+
   @override
   String get cardFilterTooltip => 'Filter';