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_en.arb

@@ -383,6 +383,20 @@
   "cardTagsEdit": "Edit tags",
   "tagChipsOverflow": "+{count, plural, =1{1 tag} other{{count} tags}}",
   "@tagChipsOverflow": {"placeholders": {"count": {"type": "int"}}},
+  "cardHints": "Hints",
+  "cardHint": "Hint {number}",
+  "@cardHint": {"placeholders": {"number": {"type": "int"}}},
+  "cardAddHint": "Add hint",
+  "cardRemoveHint": "Remove hint",
+  "cardExplanation": "Explanation",
+  "cardAddExplanation": "Add explanation",
+  "cardRemoveExplanation": "Remove explanation",
+  "cardHintsSheetTitle": "{count, plural, =1{Hint} other{Hints  {count}}}",
+  "@cardHintsSheetTitle": {"placeholders": {"count": {"type": "int"}}},
+  "cardPreviousHint": "Previous hint",
+  "cardNextHint": "Next hint",
+  "cardReport": "Report",
+  "cardSenses": "Senses",
   "cardFilterTooltip": "Filter",
   "cardFilterTitle": "Filter",
   "cardFilterTag": "Tag",