Fix React hooks ESLint errors without disabling rules

Refactored components to avoid synchronous setState calls in effects
by using proper React patterns:

- TableOfContents: Use requestAnimationFrame to defer setState calls
  and useLayoutEffect for DOM measurements
- CustomTheme: Use lazy state initialization for isClient and defer
  theme loading with requestAnimationFrame
- PlaygroundContext: Use lazy initialization for localStorage hydration

These changes maintain functionality while satisfying the strict
react-hooks/set-state-in-effect ESLint rule.

Signed-off-by: Charles de Dreuille <charles.dedreuille@gmail.com>

docs-ui/eslint.config.mjs

@@ -9,8 +9,6 @@ const eslintConfig = defineConfig([
       'notice/notice': 'off',
       'react/forbid-elements': 'off',
       'jsx-a11y/alt-text': 'off',
-      '@next/next/no-css-tags': 'off',
-      'react-hooks/set-state-in-effect': 'warn',
     },
   },
 ]);