Add `useSiwcQueryParamOptIn`, which flips on the ChatGPT sign-in rollout
localStorage flag when `?siwc-enabled=1` is present, and call it from
both pages/sign-in.tsx and pages/sign-up.tsx.
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **New Features**
* Added support for enabling the sign-in experience via
`?siwc-enabled=1`, automatically updating the stored opt-in flag on both
sign-in and sign-up pages.
* **Tests**
* Added coverage confirming the stored flag is updated only for
`siwc-enabled=1`, and not for missing, non-`1`, `0`, or repeated/array
values.
* Added assertions that the behavior is triggered consistently when
rendering the sign-in and sign-up pages.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
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Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>
Co-authored-by: Joshen Lim <joshenlimek@gmail.com>
