Synchronous dispatch() now converts office documents via OnlyOffice
(asyncio.run around OOConversionManager.convert), refusing to run inside
a live event loop and pointing async callers at
pool.generate_office_preview(). The office import stays lazy so the base
install works without the 'office' extra.
Unknown file types now name the offending extension (or its absence)
instead of a generic 'preview unsupported'.
