Codex Desktop: make the projectless voice workspace root configurable

Summary

Opening a voice conversation without an assigned project automatically creates a local workspace under a fixed default root:

${HOME}/Documents/Codex/YYYY-MM-DD/<conversation-name>

For example, this voice conversation created the date directory and then its session workspace immediately when the voice session began.

Problem

The default root is not configurable in the Codex Desktop App. On machines where the application or user account cannot write to Documents/Codex—for example because Documents is redirected, managed, read-only, unavailable, or restricted by policy—the workspace creation can fail and block voice/projectless conversations.

Observed behavior

• A new projectless voice conversation creates a date-based workspace automatically.
• The workspace contains work and outputs directories.
• Local state records the generated projectless workspace root but exposes no user-configurable setting for the default root.

Expected behavior

Provide a setting (or a supported environment/configuration option) for the default projectless/voice workspace root. Also show a clear, actionable error if the configured or default location is not writable.

Suggested resolution

1. Add a configurable projectless workspace root in Desktop settings.
2. Validate write access before starting a voice conversation.
3. Offer a fallback location or a folder picker when the default location is unavailable.
4. Preserve the current date/session subdirectory organization beneath the selected root.