Feature Request: Allow custom Python version selection per independent instance & fully isolate Python env for each instance

Expected Feature
1.Add a setting panel inside each instance's Manage page, supporting two modes:
1.1Auto bundled Python: Keep the original uv auto-install logic (default option).
1.2Custom Python path: Let users select an existing system Python executable or a pre-built venv folder for this instance only.
2.Each instance’s Python environment is strictly isolated, never cross-contaminate packages between instances.

Start at the per-instance Manage page and trace the existing uv auto-install logic. Done means each instance offers bundled Python or a selected system executable or pre-built venv, with package environments remaining isolated between instances.

Written by the indexing model from the issue text.