feat(web): add theme system with CSS variables and settings modal (#4133)

- Add ThemeContext with light/dark/system theme support
- Migrate all hardcoded colors to CSS variables
- Add SettingsModal for theme customization
- Add font loader for dynamic font selection
- Add i18n support for Chinese and Turkish locales
- Fix accessibility: add aria-live to pairing error message

Co-authored-by: nanyuantingfeng <nanyuantingfeng@163.com>

web/src/hooks/useTheme.ts

@@ -0,0 +1,4 @@
+import { useContext } from 'react';
+import { ThemeContext } from '../contexts/ThemeContextDef';
+
+export const useTheme = () => useContext(ThemeContext);