Generalise the Qubes PDF Converter to Other Types of Files

The Qubes PDF converter safely sanitises untrusted PDFs by rendering them inside a disposable VM. However, other risky file types (documents, spreadsheets, images) lack equivalent protection. This project generalises the converter into a plugin-based architecture with per-format qrexec services and minimal DispVM templates. Key features: • New qvm-convert-file entry point with MIME-based routing • Document & spreadsheet conversion via LibreOffice → existing bitmap pipeline • Image sanitisation via decode + re-encode (metadata stripped) • Audio/video conversion via ffmpeg (stretch goal) • Full backward compatibility with qvm-convert-pdf Deliverables: D1: Plugin architecture + MIME dispatch D2–D3: Document & spreadsheet converters D4: Image sanitiser D5: Media converter (stretch)