Add conda package manager support to environment setup

[jeremymanning]

Problem Description

The widget interface includes conda as a package manager option in the Advanced Options dropdown, but conda support is not actually implemented in the backend.

Current State

• Widget dropdown shows: ["auto", "pip", "conda", "uv"]
• Many default configurations specify "package_manager": "conda"
• Backend get_package_manager_command() handles:
  • ✅ "uv" → "uv pip"
  • ✅ "auto" → "uv pip" if available, else "pip"
  • ✅ "pip" → falls through to "pip" (works correctly)
  • ❌ "conda" → falls through to "pip" (ignores user choice)

Expected Behavior

When "conda" is selected as package manager:

1. Use conda for package installation instead of pip
2. Properly handle conda environment creation and activation
3. Support conda-specific package resolution and installation
4. Maintain compatibility with existing conda environments

Implementation Requirements

• Update get_package_manager_command() to handle "conda" case
• Add conda environment setup logic (instead of venv)
• Add conda package installation commands
• Add conda availability detection
• Add comprehensive tests for conda functionality
• Update documentation with conda usage examples

Files to Modify

• clustrix/utils.py - Core package manager logic
• clustrix/config.py - Configuration validation if needed
• Tests and documentation

Use Cases

• HPC environments where conda is standard
• Data science workflows requiring conda-forge packages
• Environments with complex dependency requirements
• Systems where pip+venv may have limitations

Priority: Medium - Affects user experience when conda is selected but not properly used.

Note: pip and uv support are already working correctly.