fix: chunk large file manager uploads

[ZacharyZcR]

Summary

• upload files at or above 512MiB through raw 16MiB chunks instead of one multipart/FormData request
• add a file-manager chunk upload endpoint that writes each chunk to the remote SFTP file at the supplied byte offset
• keep the existing multipart streaming path for smaller uploads

Refs Termix-SSH/Support#870

Validation

• npm run type-check
• npx eslint src/backend/ssh/file-manager-content-routes.ts src/ui/api/ssh-file-operations-api.ts
• git diff --check

Note: npm run build still fails in this checkout because @fontsource/jetbrains-mono/400-italic.css is missing from the local install; the failure occurs before this change's backend build step.