~anthropic/claude-sonnet-latest → B (85:15)

Side B implements substantive functionality: server-side theme persistence via cookies (replacing pure client-side localStorage), room-aware URL rewriting for RPC/API responses (item_path_for_api_in_room, forum_thread_web_url), a new /theme POST route, and updated CSS/tests to match. Side A is purely a rustfmt/formatting pass with no behavioral change, which has minimal lasting value beyond style consistency.