feat(app): quiz vocabulary as multiple choice

The session settings' new Vocabulary section quizzes words as multiple
choice: their meaning (the default), their reading, the expression for
the meaning, or the reading and the meaning in turn ("Mixed"). The
wrong answers come from the other words studied; none of the word's
own meanings counts as wrong, whatever its case. A word without what is
asked, or without two others to draw from, stays a word to flip. It is
asked the same way each time it comes, and rated as any card.

The settings keep both (`quizVocabulary`, `quizAspect`; settings schema
5), on every device.

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

@@ -635,6 +635,14 @@
   "studyModeSpacedHint": "Due cards on their schedules; ratings move them.",
   "studyModeCram": "Cram",
   "studyModeCramHint": "Walk every card without touching the schedules.",
+  "studySettingsVocabulary": "Vocabulary",
+  "studySettingsQuizMc": "Quiz with multiple choice",
+  "studySettingsQuizMcDescription": "Show vocabulary cards as a multiple-choice question, with wrong answers drawn from other vocabulary in this set.",
+  "studyAspectMeaning": "Meaning",
+  "studyAspectReading": "Reading",
+  "studyAspectExpression": "Expression",
+  "studyAspectMixed": "Mixed",
+  "studySettingsAspectDescription": "What to recall — the meaning, the kana reading, or the expression itself (shown the meaning). Mixed varies meaning and reading per card; cards that can’t be quizzed stay a normal flip.",
   "studySortOldestFirst": "Oldest first",
   "studySortRandom": "Random",
   "studySortNewestFirst": "Newest first",