feat(app): stroke order for kanji and kana, from KanjiVG

KanjiVG's strokes come with the app (assets/kanjivg/, CC BY-SA 3.0, its
notice in the licence list): 94 gzipped files, one per 256 code points,
2.7 MB in all, a file read the first time a character of it is asked for
(StrokeRepository, app-wide beside the maps). The strokes are SVG path
data, read with path_parsing (the Flutter team's, pure Dart, already in
the build through flutter_svg), so the model can flatten them into points
without dart:ui, as checking handwriting will.

A character's sheet traces its strokes in order over those to come, with
its stroke count, meanings, readings, level and the components KanjiVG
splits it into, each a tap away. It opens from a studied character's
answer (Stroke order) and from a character found in the dictionary.

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

@@ -593,6 +593,10 @@
   "@dictionaryConfirmCharactersTitle": {"placeholders": {"count": {"type": "int"}}},
   "dictionaryCharacterCredits": "Characters from {sources}.",
   "@dictionaryCharacterCredits": {"placeholders": {"sources": {"type": "String"}}},
+  "kanjiStrokeOrder": "Stroke order",
+  "kanjiPlayAgain": "Play again",
+  "kanjiNoStrokes": "There's no stroke order for this character.",
+  "kanjiComponents": "Components",
   "aboutDictionaries": "Dictionaries on this instance",
   "cardReportTitle": "Report an error",
   "cardReportSubtitle": "Flag this card so you can find and fix it later.",