Add Settings voice picker for Realtime calls

[g-baskin]

Feature request

Add a Settings view with an assistant voice picker for Realtime calls.

Why

Brah currently uses a hardcoded Realtime voice (marin) in the main-process session defaults. Users who want a different assistant personality have to edit source and restart the app, which is inconvenient and not discoverable.

Suggested behavior

• Add a Settings entry in the app menu.
• Show an "Assistant voice" dropdown with supported OpenAI Realtime built-in voices.
• Persist the selected voice in Electron userData.
• Apply the selected voice when creating the next Realtime client secret / call session.
• Keep the existing default (marin) for new installs.

Notes

The current OpenAI Realtime built-in voice list includes alloy, ash, ballad, coral, echo, sage, shimmer, verse, marin, and cedar; OpenAI recommends marin and cedar for best quality.