Improve workflow assets and document table interactions

From the PR description

Summary

• migrate workflow assets onto the standard document and document-version model
• add versioned asset upload, saved-file import, viewing, and consistent Office preview conversion
• refine workflow/add-on modals, prompt editor loading and table insertion, and document table selection/toolbar behavior
• keep add-on imports on the catalog page with a plain row action and completed state

Testing

• npm test --prefix backend
• npm test --prefix frontend
• npm run build --prefix backend
• npm run build --prefix frontend
• npm run typecheck --prefix word-addin
• npm run lint --prefix frontend (0 errors; existing warnings only)
• git diff --check