feat: prepare react 19 compatibility

[noahwaldner]

Prepares the packages components, icons and tokens to be compatible with react 19

• Added CI script that runs a v19 typecheck on the repository to flag additions that would break v19 compatibility (while fondue uses v18 for development)

Code changes for v19 compatibility

• useRef() without argument → explicit initial value
  @types/react@19 will make the argument required.

useRef<ReactNode>()  →  useRef<ReactNode>(undefined)

• Ref-accepting parameters: RefObject<T> → RefObject<T | null>
  In react 19 useRef<T>(null) returns RefObject<T | null>; parameters must accept it. Backward-compatible in v18.

(ref: RefObject<HTMLElement>)  →  (ref: RefObject<HTMLElement | null>)

• Refs in output positions: RefObject<T> → structural { current: T | null }
  Same nullability, but in return types/props the generic spelling fails react 18's covariance check, so the plain structural type is the only form valid under both.

triggerListRef: RefObject<HTMLDivElement>  →  triggerListRef: { current: HTMLDivElement | null }

- Duck-typed element narrowing → isValidElement<Props>()
react 19 changes ReactElement's default props from any to unknown, breaking .props access after manual checks.

typeof children === 'object' && children && 'props' in children  // then children.props.className
→  isValidElement<{ className?: string }>(children)

• Bare JSX.Element annotations → ReactElement
  react 19 removes the global JSX namespace (now React.JSX / importable only).

(): JSX.Element =>  →  (): ReactElement

• Removed ReactSVG type → self-contained local union
  react 19 deleted ReactSVG; its replacement SVGElementType in 18, and the type is publicly exported, so neither import works for both majors.

keyof ReactSVG  →  SVGElementTagName  // local union mirroring 19's SVGElementType

• Public exports typed as raw (props, ref) functions → typed as the forwardRef result
  react 19's FunctionComponent lost its legacy context?: any second parameter, so two-argument functions are no longer valid component types

export const TextInput: typeof TextFieldRoot & {…}  →  typeof ForwardedRefTextFieldRoot & {…}

[changeset-bot]

🦋 Changeset detected

Latest commit: 019f33b

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
@frontify/fondue-components	Patch
@frontify/fondue-tokens	Patch
@frontify/fondue-icons	Patch
@frontify/fondue	Patch

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