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_en.arb

@@ -266,6 +266,23 @@
   "studysetSpacedRepetition": "Spaced repetition",
   "studysetSpacedRepetitionHint": "Review the cards that are due, scheduled by FSRS.",
   "studysetImportContent": "Import content",
+  "studysetMenuExport": "Export content",
+  "exportRunning": "Exporting…",
+  "exportedMarkdown": "Exported as Markdown.",
+  "exportedZip": "{count, plural, =1{Exported as Markdown with 1 file (.zip).} other{Exported as Markdown with {count} files (.zip).}}",
+  "@exportedZip": {"placeholders": {"count": {"type": "int"}}},
+  "exportMissingFiles": "{count, plural, =1{1 file wasn't on this device and is left out.} other{{count} files weren't on this device and are left out.}}",
+  "@exportMissingFiles": {"placeholders": {"count": {"type": "int"}}},
+  "exportFailed": "Couldn't export the studyset.",
+  "exportQuestion": "Q",
+  "exportAnswer": "A",
+  "exportHint": "Hint",
+  "exportIfRight": "If right",
+  "exportIfWrong": "If wrong",
+  "exportSound": "Sound",
+  "exportVideo": "YouTube video",
+  "exportSpoiler": "Spoiler",
+  "exportUnavailable": "not available on this device",
   "studysetImportPath": "Learning path",
   "studysetImportPathHint": "From a Halkyon path JSON file",
   "studysetImportAnki": "Anki deck",