fix(postinstall): add venv creation fallback for Debian/Ubuntu

[lixiao888]

Problem

npm install -g @konbakuyomu/smart-search@latest fails on Debian/Ubuntu systems with:

The virtual environment was not created successfully because ensurepip is not
available. On Debian/Ubuntu systems, you need to install the python3-venv
package using the following command.

Root cause: The postinstall.js script uses python3 -m venv which requires the ensurepip module. On Debian/Ubuntu, python3-venv is not installed by default, so ensurepip is missing.

Fix

Add a three-strategy fallback chain for venv creation:

1. python3 -m venv — original behavior, works when python3-venv is installed
2. virtualenv — install via pip install --user virtualenv if missing, then use it (bundles pip independently, no ensurepip needed)
3. venv --without-pip + get-pip.py — create venv without pip, then bootstrap pip by downloading get-pip.py from bootstrap.pypa.io

Each strategy is tried in order; on failure, the partial venv is cleaned up before trying the next. If all strategies fail, a clear error message directs the user to install python3-venv.

Testing

Verified on a Debian system without python3-venv:

• Strategy 1 fails (as expected, ensurepip missing)
• Strategy 2 skipped (virtualenv not installed)
• Strategy 3 succeeds: venv --without-pip + get-pip.py creates a working venv with pip

Changes

• npm/scripts/postinstall.js: Extract venv creation into createVenv() with three fallback strategies; wrap main logic in async main() to support get-pip.py download; improve error messages for Debian/Ubuntu users.