feat(code): enable mid-session model switching for cloud runs

[VojtechBartos]

Problem

Model dropdown didn't work on cloud runs — cloud sessions never had a model config option populated, and the click handler was gated on a status cloud runs never reach.

Solution

Renderer-only fix. No backend or sandbox changes needed (they already accept set_config_option as a passthrough).

• Fetch model/effort options from the existing preview-config trpc endpoint and merge them into cloud sessions' configOptions.
• Relax the ModelSelector guard so cloud sessions can fire the switch.

Showcase

code-model-switch.mov