feat: support conda/mamba environment detection

Backend:
- Add is_conda, is_mamba, conda_env_name fields to PythonEnvStatus
- Detect via CONDA_DEFAULT_ENV / MAMBA_DEFAULT_ENV env vars
- Resolve actual python path via which python3 before checks
- Conda-aware pip install when env is active

Frontend:
- Show conda/mamba env name badge alongside Python version

Author: Enols