fix(app): furigana stays on its word, and readings typed as learners type

Furigana:
- Text typed after a word with a reading no longer takes the reading.
- A reading over text formatted in parts (a bold kanji) is drawn once,
  over all of it, each part in its own style; in Markdown it is one
  <ruby>, the parts keeping their marks, the reading escaped.
- Readings follow the editor's text size, not the system's, and a
  linked word with a reading still opens its link.
- Anki's <ruby> keeps the formatting inside it.
- The Markdown export escapes what Markdown would read as its syntax
  and keeps blank lines inside code blocks.

Typing a reading:
- Hepburn as learners type it: onna is おんな, konnichiwa こんにちは, m
  before b, p and m is ん (shimbun); keyboards' n' and nnn still work.
- The caret stays where the person types, also in the middle.
- Macrons count both ways (ō as おう or おお); a card listing several
  readings (いく、ゆく) takes any of them; only words written with kanji
  ask for their reading.
- No autocorrect, suggestions or curly quotes in the field; Done with
  nothing typed keeps the keyboard; the lock screen lets go of the field.

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/test/feature/anki/data/anki_field_test.dart

@@ -184,6 +184,24 @@ void main() {
       {'insert': 'を読む'},
     ]);
   });
+  test('formatting inside a ruby stays, with the reading over it', () {
+    final content = ankiContent([
+      '<ruby><b>漢</b>字<rt>かんじ</rt></ruby>',
+    ], assetOf: files.get);
+    final document = content.editorJson['document']! as Map<String, Object?>;
+    final line = (document['children']! as List).single as Map;
+
+    expect((line['data']! as Map)['delta'], [
+      {
+        'insert': '漢',
+        'attributes': {'ruby': 'かんじ', 'bold': true},
+      },
+      {
+        'insert': '字',
+        'attributes': {'ruby': 'かんじ'},
+      },
+    ]);
+  });
 }
 
 extension on Map<String, String> {