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_de.dart

@@ -885,6 +885,67 @@ class AppLocalizationsDe extends AppLocalizations {
   @override
   String get studysetImportContent => 'Inhalt importieren';
 
+  @override
+  String get studysetMenuExport => 'Inhalt exportieren';
+
+  @override
+  String get exportRunning => 'Wird exportiert …';
+
+  @override
+  String get exportedMarkdown => 'Als Markdown exportiert.';
+
+  @override
+  String exportedZip(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: 'Als Markdown mit $count Dateien exportiert (.zip).',
+      one: 'Als Markdown mit 1 Datei exportiert (.zip).',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String exportMissingFiles(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: '$count Dateien waren nicht auf diesem Gerät und fehlen.',
+      one: '1 Datei war nicht auf diesem Gerät und fehlt.',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String get exportFailed => 'Das Lernset konnte nicht exportiert werden.';
+
+  @override
+  String get exportQuestion => 'F';
+
+  @override
+  String get exportAnswer => 'A';
+
+  @override
+  String get exportHint => 'Hinweis';
+
+  @override
+  String get exportIfRight => 'Wenn richtig';
+
+  @override
+  String get exportIfWrong => 'Wenn falsch';
+
+  @override
+  String get exportSound => 'Ton';
+
+  @override
+  String get exportVideo => 'YouTube-Video';
+
+  @override
+  String get exportSpoiler => 'Spoiler';
+
+  @override
+  String get exportUnavailable => 'nicht auf diesem Gerät verfügbar';
+
   @override
   String get studysetImportPath => 'Lernpfad';