feat(paths): path files carry tables, callouts and furigana, and several import at once

The import format gains the blocks lessons need: tables (rows of cells),
callouts (a kind, a title, blocks inside, but no table or callout),
spoilers, quotes and dividers; and runs of text gain a reading shown as
furigana, a colour, underline and strikethrough. A line break is a "\n"
in the text. Still simple blocks turned into rich text here, so a change
of the editor's own format never breaks a file written before. A table's
columns are as wide as their longest line, narrowed alike to fit a
phone, where the cells wrap.

A studyset's Import › Learning paths now takes several files at once,
one path each, stored in the order of their names, so a chapter's
lessons come in with one pick. Every file is read before anything is
stored: one that can't be read is named, and nothing comes in.

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/studyset/widget/studyset_screen_test.dart

@@ -428,7 +428,7 @@ void main() {
 
     await choose(tester, 'Import content');
     expect(find.text('Anki deck'), findsOneWidget);
-    expect(find.text('Learning path'), findsOneWidget);
+    expect(find.text('Learning paths'), findsOneWidget);
   });
 
   testWidgets("a studyset's More info shows its id and its parent's, to "