fix: explicitly define children prop for React 19 compatibility

[jctf]

Fixes #367

React 19 introduced changes to how PropsWithChildren works in the type system, causing TypeScript errors when using
ThemeProvider with children:

Type '{ children: Element; ... }' is not assignable to type 'IntrinsicAttributes & ThemeProviderProps'.                   
  Property 'children' does not exist on type 'IntrinsicAttributes & ThemeProviderProps'.                                  

Solution

This PR explicitly defines the children prop as React.ReactNode instead of extending PropsWithChildren<unknown>.

Changes:

- export interface ThemeProviderProps extends React.PropsWithChildren<unknown> {                                          
+ export interface ThemeProviderProps {                                                                                   
+   /** Children elements to be wrapped by the theme provider */                                                          
+   children?: React.ReactNode                                                                                            

Backward Compatibility

This change is fully backward compatible with React 16.8+, 17, and 18:

• React.ReactNode has been stable since React 16+
• Explicitly defining children?: React.ReactNode is functionally equivalent to PropsWithChildren<unknown> in older
  React versions
• All examples in this repo (using React 18.2.0) build successfully with this change
• Only type definitions changed - no runtime behaviour changes