Hands-free voice mode (VAD) + install-on-demand for voice dependencies (PR4 of 6)

[RONAK-AI647]

What this does

Two things in this PR:

1. No more manual terminal installs. The first time you try voice mode and
   the required tools (sox, whisper, piper) aren't installed, Nanocoder now asks
   you right there in the app - "Install now? Y/N" - and installs everything
   automatically if you say yes. No separate terminal, no manual steps.

2. Real hands-free voice. You can now switch to /voice hands-free and just
   talk - no need to hold a button. Nanocoder listens continuously and
   automatically detects when you start and stop speaking.

Known limitation

The hands-free listening uses a simple volume-based detection method rather than
a full AI speech detector. This keeps it lightweight and working across every
platform, but it means loud background noise could occasionally be picked up as
speech. This is a reasonable first version - a smarter version can come later if
needed.

Test 1: Install prompt

1. Make sure sox/whisper/piper are NOT installed.
2. Run /voice or Ctrl+T.
3. Confirm you see: "Voice mode requires sox, whisper, piper (~150 MB). Install now?"
4. Press Esc/No - confirm it cancels cleanly and doesn't ask again this session.
5. Restart the app, trigger voice again - confirm it DOES ask again (session-only memory).

Test 2: Real install

1. Trigger the prompt again, select Yes.
2. Confirm you see actual progress updates, not just a frozen "Installing..." screen.
3. Once done, run the same command-line check we used before
   (e.g. which sox, which whisper-cli, which piper) and confirm all three
   are genuinely present - not just sox.

Test 3: Hands-free mode

1. Run /voice hands-free.
2. Speak without pressing anything - confirm the status bar shows Listening
   automatically.
3. Stop speaking - confirm it transitions to Processing, then Speaking (reads
   the reply), then back to Listening automatically for the next turn.

Test 4: Push-to-talk still works

1. Run /voice push-to-talk.
2. Confirm Ctrl+T still works exactly as it did in PR3.

[will-lamerton]

Nice work @RONAK-AI647 :)