fix: venv detection, Windows paths, and install dir defaulting to pwd (#7)

- Default INSTALL_DIR to current working directory (pwd) instead of $HOME
- Skip venv creation if already inside an active venv ($VIRTUAL_ENV is set)
- Search for existing venv activate script before creating a new one
- Detect Windows (Git Bash/MSYS2) activate path (Scripts/ vs bin/)
- Fix hardcoded bin/activate in generated start_lmstudio_mcp.sh
- Add error trap for clearer failure messages

Fixes: install breaks out to $HOME when run from Pinokio or any
pre-configured venv environment on Windows.

Author: infinitimeless