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_de.arb

@@ -344,6 +344,18 @@
   "cardTagsAdd": "Tags hinzufügen",
   "cardTagsEdit": "Tags bearbeiten",
   "tagChipsOverflow": "+{count, plural, =1{1 Tag} other{{count} Tags}}",
+  "cardHints": "Hinweise",
+  "cardHint": "Hinweis {number}",
+  "cardAddHint": "Hinweis hinzufügen",
+  "cardRemoveHint": "Hinweis entfernen",
+  "cardExplanation": "Erklärung",
+  "cardAddExplanation": "Erklärung hinzufügen",
+  "cardRemoveExplanation": "Erklärung entfernen",
+  "cardHintsSheetTitle": "{count, plural, =1{Hinweis} other{Hinweise  {count}}}",
+  "cardPreviousHint": "Vorheriger Hinweis",
+  "cardNextHint": "Nächster Hinweis",
+  "cardReport": "Meldung",
+  "cardSenses": "Wortbedeutungen",
   "cardFilterTooltip": "Filter",
   "cardFilterTitle": "Filter",
   "cardFilterTag": "Tag",