fix(windows): clean up qpdf staging files

[terminalchai]

Summary

• Wrap Windows qpdf staging in try/finally and remove its temporary directory on success or failure.
• Preserve copied qpdf binaries, Visual C++ runtime files, existing output, and version handling.

Why

Closes #38. Downloaded archives and extracted files currently remain in offpdf-qpdf-* directories after the script exits.

Validation

• npm run build
• npm test (261 tests across 30 files)
• cargo check --manifest-path src-tauri/Cargo.toml (not run; no Rust changes)
• PowerShell syntax parsing passed.
• An isolated local verification harness reproduced the staging leak before the change with a simulated download failure.
• After the change, simulated download and extraction failures both preserved the original error and left no staging directories.
• A real qpdf 12.4.1 download/extraction completed with no staging directories left behind. The staged executable reported version 12.4.1, and all three required runtime DLL copies matched their source hashes.
• The local harness and downloaded artifacts are not included in this PR.

Privacy Checklist

• This keeps OffPDF usable offline.
• This does not upload, log, or transmit user files.
• New dependencies or bundled binaries have compatible licenses (none added).

[McanKul]

Thanks for the focused fix and thorough validation. I also verified the success and simulated failure paths locally; the staged binaries remain intact and the temporary directory is removed in both cases. Great first contribution!