Add a dark-mode theme store to the frontend

Add a dark-mode theme store to the frontend: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). The web app is built with Tailwind configured for class-based dark mode (the dark variant activates when an ancestor element carries the dark CSS class). We want a small…