feat(app): a character card for kanji and kana

One card type for both (card schema 7): the character on the front, its
meanings and a kanji's on and kun readings, each merging on its own. A
kana's romaji is its meaning, so kana are studied, and later written,
exactly as kanji are. The composer offers it beside the other kinds, each
kind of reading on one line (ショク、ジキ); studying shows the character
large and, with the answer, its meanings and readings (the other way round
when asked from the back); the card browser filters by it and finds a kun
reading without its okurigana dot; the Markdown export writes its meanings
and readings.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_en.arb

@@ -625,16 +625,19 @@
   "composerKindVocabulary": "Vocabulary",
   "composerKindMatching": "Matching",
   "composerKindMap": "Map",
+  "composerKindCharacter": "Character",
   "composerLabelFront": "Front",
   "composerLabelQuestion": "Question",
   "composerLabelExpression": "Expression",
   "composerLabelPrompt": "Prompt",
+  "composerLabelCharacter": "Character",
   "composerLabelBack": "Back",
   "composerLabelAnswers": "Answers",
   "composerLabelAnswer": "Answer",
   "composerPlaceholderFront": "Add question",
   "composerPlaceholderExpression": "Add expression",
   "composerPlaceholderPrompt": "Add prompt",
+  "composerPlaceholderCharacter": "Add a kanji or kana",
   "composerAddAnswer": "Add answer",
   "composerCorrectAnswer": "Correct answer",
   "composerWrongAnswer": "Wrong answer",
@@ -685,6 +688,13 @@
   "vocabularyRemoveGloss": "Remove translation",
   "vocabularyAddSense": "Add sense",
   "vocabularyRemoveSense": "Remove sense",
+  "characterPrimaryMeaningHint": "Primary meaning, or a kana's romaji",
+  "characterOnReadings": "On readings",
+  "characterOnReadingsHint": "From Chinese, in katakana: ショク、ジキ",
+  "characterKunReadings": "Kun readings",
+  "characterKunReadingsHint": "Japanese, in hiragana, okurigana after a dot: た.べる",
+  "characterOn": "On",
+  "characterKun": "Kun",
   "matchingCategories": "Categories",
   "matchingCategoriesHint": "The drop-zone cards learners drag items onto. Each category needs at least one item, so a card can't have more categories than items.",
   "matchingCategory": "Category {number}",
@@ -1427,6 +1437,7 @@
   "cardBrowserTypeVocabulary": "Words only",
   "cardBrowserTypeMatching": "Matching only",
   "cardBrowserTypeMap": "Maps only",
+  "cardBrowserTypeCharacter": "Characters only",
   "cardBrowserFilterTag": "Tag",
   "cardBrowserAllTags": "All tags",
   "cardBrowserFilterStatus": "Status",