install.sh times out when installing behind a proxy

When installing behind a proxy, the installation script fails:
```
$ sudo ./install.sh
Installing Time Config Hub...
Reading configuration from conf/tch_app.conf...
ConfigDirectory (for traffic configs): /etc/tch/tsn_configs
LogDirectory: /var/log/tch
================================================
Time Config Hub Installation Script
================================================

Validating project structure...
✓ Project structure validated

Creating directory structure...
✓ Directory structure created:
- Application config: /etc/tch/app_config
- TSN config: /etc/tch/tsn_configs
- YANG modules: /etc/tch/yang_modules
- Logs: /var/log/tch
- Python venv base: /opt/tch

Copying application configuration files...
✓ Application config files copied to /etc/tch/app_config/

Checking for required dependencies...
✓ libyang detected via yanglint (yanglint 4.2.2)

Installing Python package and dependencies...
Creating Python virtual environment...
✓ Virtual environment created: /opt/tch/venv
Requirement already satisfied: pip in /opt/tch/venv/lib/python3.13/site-packages (25.1.1)
WARNING: Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ConnectTimeoutError(, 'Connection to pypi.org timed out. (connect timeout=15)')': /simple/pip/
```

This can be prevented by populating the /opt/tch/venv/pip.conf file with the proxy settings (e.g. from the environment variables) like in:
```
[global]
proxy = http://proxy.example.com:8080
```

Start with install.sh and trace the Python virtual-environment setup and dependency installation. Check how proxy environment variables are available and how /opt/tch/venv/pip.conf is created; done means installation can use the configured proxy instead of timing out against pypi.org.

Written by the indexing model from the issue text.