feat(learn): an assistant explains a card missed and checks what was written

A conversation has a task (ADR 0021, D14): asking, explaining a card or
checking what was written, which the system prompt says and the history
shows. Rating a card Bad offers Explain in a snack bar lifted above the
ratings (D53); a reading typed wrong asks Why? (D54), and the field's
Check has the text checked in the studyset's language, the answer in the
app's (D54). Explaining sends the card's own parts, in a conversation
named by its front.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_flutter/lib/l10n/app_de.arb

@@ -1837,6 +1837,8 @@
   "assistantHistory": "Unterhaltungen",
   "assistantHistoryEmpty": "Noch keine Unterhaltungen",
   "assistantHistoryEmptyHint": "Frag einen Assistenten im Feld unten.",
+  "assistantHistoryExplained": "Karte erklärt",
+  "assistantHistoryChecked": "Auf Fehler geprüft",
   "assistantHistoryDelete": "Unterhaltung löschen",
   "assistantHistoryDeleteMessage": "„{title}“ verschwindet von allen deinen Geräten.",
   "assistantHistoryDeleteConfirm": "Löschen",
@@ -1865,6 +1867,12 @@
   "assistantPrivateMark": "{name} darf dieses private Lernset bekommen",
   "studysetPrivacyAssistants": "Assistenten, an die es gehen darf",
   "studysetPrivacyTakeBack": "Zurücknehmen",
+  "studyMissedExplain": "Nicht gewusst? {name} kann erklären, warum.",
+  "assistantExplain": "Erklären",
+  "assistantExplainQuestion": "Warum ist das die Antwort?",
+  "studyTypedWhy": "Warum?",
+  "assistantWhyTyped": "Ich habe „{typed}“ getippt. Warum stimmt das nicht?",
+  "assistantCheck": "Auf Fehler prüfen",
   "settingsSearchEngine": "Suchmaschine",
   "searchEngineDevice": "Suche des Geräts",
   "searchEngineCustom": "Eigene Adresse",