Add dark mode toggle

## Description
The timetable UI currently only supports a light theme. Adding a dark/light mode toggle will allow users to switch themes, improving accessibility and overall user experience.

## Steps to Implement
1. Add a toggle button in the UI (e.g., in the header or settings panel).
2. Apply dark mode styles to all components (backgrounds, text, buttons, tables, etc.).
3. Store user preference (e.g., using localStorage or React context) to remember the selected theme.

## Expected Behavior
- Users can switch between light and dark themes seamlessly.
- All UI elements adapt correctly to the selected theme.

尋找主要的 UI 元件，它們可能位於頁首或設定面板中，以新增切換按鈕。使用 React context 或 localStorage 管理主題狀態。透過檢查現有的 CSS 或樣式函式庫，為所有元件套用深色模式樣式。確認主題能夠持續保存，且所有 UI 元素都能正確適應。

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