feat: add notification sounds with settings toggle

[moehaje]

Summary

• Add notification sounds for long‑running agent completions and errors when the app is unfocused.
• Add Display & Sound settings section with a notification toggle and a test sound button.
• Add reliable window focus tracking to gate notifications.
• Bundle success/error sound assets and set volume to 5%.

Details

• Sounds are bundled via src/assets and played through a shared helper.
• Notifications trigger after 60s+ of agent work (unfocused), with fallback event paths.
• Settings are persisted in app settings.

Testing

• pnpm typecheck
• Manual: Settings → Display & Sound → Test sound (alternates success/error), verify unfocused behavior.