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/lib/l10n/app_localizations_en.dart

@@ -387,6 +387,23 @@ class AppLocalizationsEn extends AppLocalizations {
     return 'Couldn\'t import this file. $reason';
   }
 
+  @override
+  String pathImportFileFailed(String file, String reason) {
+    return 'Couldn\'t import $file. $reason';
+  }
+
+  @override
+  String pathsImported(int count, int steps) {
+    String _temp0 = intl.Intl.pluralLogic(
+      steps,
+      locale: localeName,
+      other: 'Imported $count paths with $steps steps.',
+      one: 'Imported $count paths with 1 step.',
+      zero: 'Imported $count paths without steps.',
+    );
+    return '$_temp0';
+  }
+
   @override
   String get pathImportUnreadable => 'It isn\'t text in UTF-8.';
 
@@ -1043,10 +1060,11 @@ class AppLocalizationsEn extends AppLocalizations {
   String get exportUnavailable => 'not available on this device';
 
   @override
-  String get studysetImportPath => 'Learning path';
+  String get studysetImportPath => 'Learning paths';
 
   @override
-  String get studysetImportPathHint => 'From a Halkyon path JSON file';
+  String get studysetImportPathHint =>
+      'From Halkyon path JSON files, one path each';
 
   @override
   String get studysetImportAnki => 'Anki deck';
@@ -4851,6 +4869,20 @@ class AppLocalizationsEn extends AppLocalizations {
   String get pathImportMalformedSource =>
       'a source must be an object or text, and its parts text.';
 
+  @override
+  String get pathImportMalformedTable =>
+      'a table needs \"rows\" of cells, each as long as the first.';
+
+  @override
+  String pathImportMalformedCallout(String detail) {
+    return 'a callout with a malformed $detail.';
+  }
+
+  @override
+  String pathImportNotInCallout(String detail) {
+    return 'a $detail can\'t be inside a callout.';
+  }
+
   @override
   String get pomodoroTopBarHint =>
       'The timer can count down in GNOME\'s top bar, with its buttons, once the AppIndicator extension is on.';