feat: support CLI default model option

[TeigenZhang]

Summary

• Add a "Default (CLI default)" option to the model selection dropdown, allowing sessions to inherit the CLI's own default model instead of forcing a specific one
• Treat empty string as undefined when passing model to Session, applied consistently across session creation, quick-start, and Ralph

Test plan

• Open Settings → Model Config → verify "Default (CLI default)" appears as first option
• Select "Default (CLI default)", create a new session → verify no --model flag is passed to CLI
• Select a specific model (e.g. Opus), create session → verify --model opus is passed
• Quick-start a session with default model → verify same behavior
• Start a Ralph loop with default model → verify no model override

🤖 Generated with Claude Code

[Ark0N]

Thanks for the contribution @TeigenZhang! Clean, well-scoped change. The || undefined guards are the right approach to ensure empty string doesn't pass through as a model name.