Add an "Export" item to the row context menu with a submenu:
- a folder offers ".zip" (reuses download.downloadFolder; works offline + server)
- an md/docx/html file offers the OTHER two formats, each triggering a
server-side conversion download via the new download.exportFile (builds the
sibling-extension URL and lets the browser pull the converted bytes). File
conversion is server-only, so it's hidden in offline (FS) mode; a zip is
already an archive and gets no Export.
menu-model's toMenuItem now passes a descriptor's `items` through as a submenu
(resolved against the captured browse ctx) instead of only emitting action rows.
Verified: 11/11 browse Playwright specs pass (incl. menu/context + Download ZIP);
a logic harness confirms the per-type submenu contents and that clicks route to
download.exportFile / downloadFolder.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
ZDDC
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