Allow RDP connections without stored credentials

[ZacharyZcR]

Summary

• set Guacamole disable-auth for RDP tokens when no username or password is configured
• keep normal RDP credential authentication unchanged
• add token-service coverage for empty and populated RDP credentials

Verification

• npm run type-check
• npx eslint src/backend/guacamole/token-service.ts src/backend/guacamole/token-service.test.ts
• npx prettier --check src/backend/guacamole/token-service.ts src/backend/guacamole/token-service.test.ts
• git diff --check

Notes

• npx vitest run src/backend/guacamole/token-service.test.ts could not start because the local install does not resolve vitest/config.
• npm run build still fails before this change is reached because @fontsource/jetbrains-mono/400.css cannot be resolved from src/ui/index.css.

Fixes Termix-SSH/Support#864