feat(app): export a studyset as Markdown

Export content, in the studyset's options, writes the studyset and
everything inside it as one readable Markdown document (decided
2026-10-08: no re-importable archive before the public tier's release
files): its title, secondary title, description and licence, then its
flashcards (answers below their questions, choices as task lists),
notes, learning paths (steps with their kinds, answers, feedback and
sources), PDFs and bibliography, then each sub-studyset a level deeper.
Rich text keeps its formatting; formulas go between dollars, callouts
become quotes, spoilers HTML details, YouTube videos links. With files
(images, sounds, PDFs) it keeps a .zip of the document and them under
files/; a file this device can't have is named and counted.

Saving goes through a FileSaver: file_selector's dialog on the desktop,
file_picker's (Android's Storage Access Framework) on phones, a direct
dependency now at the version the editor already brings.

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

@@ -879,6 +879,67 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get studysetImportContent => 'Import content';
 
+  @override
+  String get studysetMenuExport => 'Export content';
+
+  @override
+  String get exportRunning => 'Exporting…';
+
+  @override
+  String get exportedMarkdown => 'Exported as Markdown.';
+
+  @override
+  String exportedZip(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: 'Exported as Markdown with $count files (.zip).',
+      one: 'Exported as Markdown with 1 file (.zip).',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String exportMissingFiles(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: '$count files weren\'t on this device and are left out.',
+      one: '1 file wasn\'t on this device and is left out.',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String get exportFailed => 'Couldn\'t export the studyset.';
+
+  @override
+  String get exportQuestion => 'Q';
+
+  @override
+  String get exportAnswer => 'A';
+
+  @override
+  String get exportHint => 'Hint';
+
+  @override
+  String get exportIfRight => 'If right';
+
+  @override
+  String get exportIfWrong => 'If wrong';
+
+  @override
+  String get exportSound => 'Sound';
+
+  @override
+  String get exportVideo => 'YouTube video';
+
+  @override
+  String get exportSpoiler => 'Spoiler';
+
+  @override
+  String get exportUnavailable => 'not available on this device';
+
   @override
   String get studysetImportPath => 'Learning path';