Replace deprecated React.FormEvent with SubmitEvent (#99821)
## Summary
`@types/react` 19.2.10 deprecates `FormEvent`/`FormEventHandler` (they
don't correspond to a real DOM event). This switches `onSubmit` handlers
to `SubmitEvent<HTMLFormElement>` in test fixtures, the docs
(`authentication.mdx`, `preserving-ui-state.mdx`), and
`examples/with-supabase`. It also removes an unused `FormEvent` import
from a JSX docs sample.
Examples still pinned to `@types/react` 18 are unchanged, since
`SubmitEvent` only exists in `@types/react` >= 19.2.10.
## Verification
- Type-checked the updated fixtures and the docs snippet patterns with
`tsc` (strict) against `@types/react` 19.2.18, plus a negative check
confirming the event type is enforced against `onSubmit`. The fixture
files aren't in the root `tsconfig.json` include list, so `pnpm
typescript` doesn't cover them.
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