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_de.arb

@@ -240,6 +240,21 @@
   "studysetSpacedRepetition": "Verteilte Wiederholung",
   "studysetSpacedRepetitionHint": "Wiederhole die fälligen Karten, geplant durch FSRS.",
   "studysetImportContent": "Inhalt importieren",
+  "studysetMenuExport": "Inhalt exportieren",
+  "exportRunning": "Wird exportiert …",
+  "exportedMarkdown": "Als Markdown exportiert.",
+  "exportedZip": "{count, plural, =1{Als Markdown mit 1 Datei exportiert (.zip).} other{Als Markdown mit {count} Dateien exportiert (.zip).}}",
+  "exportMissingFiles": "{count, plural, =1{1 Datei war nicht auf diesem Gerät und fehlt.} other{{count} Dateien waren nicht auf diesem Gerät und fehlen.}}",
+  "exportFailed": "Das Lernset konnte nicht exportiert werden.",
+  "exportQuestion": "F",
+  "exportAnswer": "A",
+  "exportHint": "Hinweis",
+  "exportIfRight": "Wenn richtig",
+  "exportIfWrong": "Wenn falsch",
+  "exportSound": "Ton",
+  "exportVideo": "YouTube-Video",
+  "exportSpoiler": "Spoiler",
+  "exportUnavailable": "nicht auf diesem Gerät verfügbar",
   "studysetImportPath": "Lernpfad",
   "studysetImportPathHint": "Aus einer Halkyon-Pfad-JSON-Datei",
   "studysetImportAnki": "Anki-Deck",