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_server/lib/src/generated/review/review_log.dart

@@ -92,6 +92,7 @@ abstract class ReviewLog
 
   /// How the card was studied (ADR 0004): 'review' (front to back), 'reverse'
   /// (back to front, with a schedule of its own, as Anki's reverse cards have),
+  /// 'write' (written by hand, stroke by stroke, with a schedule of its own),
   /// or 'cram' (outside the schedule: counts for streaks and the activity
   /// heatmap, never moves a schedule). Reviews from before modes are 'review'.
   /// 'reset' is no rating but a fresh start: the card's schedules replay only
@@ -311,6 +312,7 @@ class ReviewLogTable extends _is.Table<_is.UuidValue?> {
 
   /// How the card was studied (ADR 0004): 'review' (front to back), 'reverse'
   /// (back to front, with a schedule of its own, as Anki's reverse cards have),
+  /// 'write' (written by hand, stroke by stroke, with a schedule of its own),
   /// or 'cram' (outside the schedule: counts for streaks and the activity
   /// heatmap, never moves a schedule). Reviews from before modes are 'review'.
   /// 'reset' is no rating but a fresh start: the card's schedules replay only

halkyon_learn_server/lib/src/review/review_log.spy.yaml

@@ -18,6 +18,7 @@ fields:
   durationMs: int
   ### How the card was studied (ADR 0004): 'review' (front to back), 'reverse'
   ### (back to front, with a schedule of its own, as Anki's reverse cards have),
+  ### 'write' (written by hand, stroke by stroke, with a schedule of its own),
   ### or 'cram' (outside the schedule: counts for streaks and the activity
   ### heatmap, never moves a schedule). Reviews from before modes are 'review'.
   ### 'reset' is no rating but a fresh start: the card's schedules replay only