fix(file-manager): make bulk uploads resilient

[ZacharyZcR]

Summary

• cap simultaneous browser uploads at three per selection
• remove the Node request-duration ceiling and extend file-manager proxy timeouts
• destroy aborted SFTP streams and remove partial remote files
• only report streamed uploads as complete after the writable stream finishes

Verification

• npx vitest run --project frontend src/ui/tests/features/file-manager/file-manager-utils.test.ts
• npx vitest run --project backend src/backend/tests/hosts/file-manager/content-routes.test.ts
• npm run lint -- --quiet
• npm run type-check

Fixes Termix-SSH/Support#1231