feat(app): write kanji, kana and words by hand, each stroke checked

The study option Write by hand (settings schema 7) has character cards
and words written in a session, on a schedule of their own (review mode
`write`; older apps skip a mode they don't know): what to write on top, a
character's meanings and readings or a word's reading and meanings, and
below it a square to write it in, character by character.

Each stroke is checked as it ends against KanjiVG's: both resampled to 22
points, then their shape once centred, where they lie, how long they are
and which way they go, each against what a loose hand strays by; a stroke
closer to one still to come is out of order. One that counts is drawn as
KanjiVG draws it, and the hand's offset from the middle is forgiven in the
strokes after; any other flashes red, says why and counts as a mistake, as
does a hint. Two misses in a row show the next stroke faintly. Once
written, or given up on (Show it), the rating the mistakes suggest is
outlined: none Good, one or two OK, more Bad.

The square takes every touch on it at once, so writing never scrolls the
card or swipes to rate. Characters without a stroke order (in a word) are
written for the person.

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_localizations_de.dart

@@ -1962,6 +1962,49 @@ class AppLocalizationsDe extends AppLocalizations {
   @override
   String get kanjiComponents => 'Bestandteile';
 
+  @override
+  String get studySettingsCharacters => 'Kanji & Kana';
+
+  @override
+  String get studySettingsWriteByHand => 'Von Hand schreiben';
+
+  @override
+  String get studySettingsWriteByHandDescription =>
+      'Kanji, Kana und Wörter schreibst du Strich für Strich, jeder Strich wird gleich geprüft, nach einem eigenen Plan. Geschriebene Wörter werden weder abgefragt noch getippt.';
+
+  @override
+  String get writingHint => 'Tipp';
+
+  @override
+  String get writingShow => 'Zeigen';
+
+  @override
+  String writingMistakes(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: '$count Fehler',
+      one: '1 Fehler',
+      zero: 'Keine Fehler',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String get writingWrongOrder => 'Noch nicht: Dieser Strich kommt später.';
+
+  @override
+  String get writingWrong => 'Das ist nicht der nächste Strich.';
+
+  @override
+  String writingSuggested(String rating) {
+    return '$rating vorgeschlagen';
+  }
+
+  @override
+  String get writingNoStrokes =>
+      'Für diese Schriftzeichen gibt es keine Strichfolge: Bewerte, wie gut du sie wusstest.';
+
   @override
   String get aboutDictionaries => 'Wörterbücher dieser Instanz';