openai/gpt-chat-latest → A (10:1)

Side A introduces reusable reducer helpers for chronological thread indices, fixes multiple call sites to use them consistently, adds guards so empty private room gardens return 404 instead of falling back to public content, documents important reducer invariants, and adds regression/integration tests covering these behaviors. Side B only moves the error container element within the new-thread form markup, a minor UI/layout adjustment with far less lasting impact.