fix(office): enable export render fallback on macOS/Linux

[kichy-ge]

Summary

• add non-COM export fallback path in cli_anything/office/core/export.py
• use office_backend (libreoffice-headless) when WPS COM is unavailable
• support writer export render on macOS/Linux (pdf, docx, txt, html)
• support calc export render on macOS/Linux (xlsx, pdf-calc) via CSV bridge
• keep impress unsupported in fallback path with explicit error message

Why

Current export render hard-fails outside Windows/WPS COM even though the repository already has a cross-platform LibreOffice backend.

Validation

Executed locally on macOS with Python 3.12 + LibreOffice:

• cli-anything-office --project demo-report.wps.json export render demo-render-patched.pdf -p pdf --overwrite
• cli-anything-office --project demo-report.wps.json export render demo-render-patched.docx -p docx --overwrite
  Both succeeded with method: libreoffice-headless.

Notes

• this PR is intentionally minimal and touches only one file
• generated demo artifacts are not included in this PR