fix: replace JSX.Element with React types for React 19 compatibility

[tyler-reitz]

Problem

Since React 19 removed `JSX.Element` from the global `JSX` namespace, components in reactfire that return `JSX.Element` no longer type-check with React 19 consumers:

```
Type error: 'AuthProvider' cannot be used as a JSX component.
Its type '(props: PropsWithChildren<{ sdk: Auth; }>) => Element' is not a valid JSX element type.
Type 'Element' is not assignable to type 'ReactNode | Promise'.
```

Fix

Replace all `JSX.Element` annotations with explicit `React.*` types. Changed in 4 places:

• `src/sdk.tsx` — `SdkProvider` return type → `React.ReactElement`
• `src/auth.tsx` — `AuthCheck` return type → `React.ReactElement`
• `src/performance.tsx` — `SuspenseWithPerf` return type → `React.ReactElement`
• `src/storage.tsx` — `INTERNALStorageImage` return type and `placeHolder` prop → `React.ReactNode` (appropriate since a placeholder can be a string, number, null, etc.)

Also adds a CI `type-check` matrix job that runs `tsc` against both `@types/react@18` and `@types/react@19` to prevent regressions.

Fixes #637

[gemini-code-assist]

Code Review

This pull request updates several component return types and prop types from JSX.Element to React.ReactElement across auth.tsx, performance.tsx, sdk.tsx, and storage.tsx. The review feedback suggests using React.ReactNode instead of React.ReactElement for the placeHolder prop in storage.tsx to allow for more flexible types like strings or numbers, and consequently updating the return type of INTERNALStorageImage to React.ReactNode as well.

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[armando-navarro]

The PR description might be a little misleading now with regard to the storage change, which becomes the squash merge commit body. Very minor, but might want to update that before merging.

One possible follow-up issue I found is that CI only type-checks against @types/react 18, so nothing stops a global JSX.Element from creeping back in and re-breaking React 19 consumers. A small CI step that packs the build and runs tsc over a fixture with @types/react@19 (and 18) would lock this fix in.

Approving the PR.