fix(smart-install): voice venv had no pip (deps silently never installed)

On Ubuntu 24.04 `python3 -m venv` produces a pip-less venv without python3-venv,
so /opt/voice-onboard/.venv/bin/pip was missing and the heavy voice deps
(pipecat/whisper/kokoro/piper) silently failed — masked by `|| true` + `| tail`,
so the step reported OK while the voice module was left non-functional (models
downloaded, runtime libs absent).

Fix: add python3-venv + python3-pip to the deps step; idx95 falls back to
--without-pip + `ensurepip --upgrade` and is now warn_if_fail (heavy/optional —
must not silently lie, must not abort studio). Functional voice still needs a
voice-capable machine (the KVM VM is headless); to be validated on real HW.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
