- Add planning model selection UI to PlanningModeModal initial view with model loading, favorites wiring, and default/custom badge state
- Extend startPlanningStreaming client API to send optional planningModelProvider/planningModelId overrides
- Propagate planning model overrides through /planning/start-streaming and createSessionWithAgent so agent creation uses selected model settings
- Add frontend and backend tests covering modal selection behavior, request forwarding, and end-to-end planning model flow
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