refactor(core): use atob and btoa instead of Buffer

[andrascodes]

This PR is part of a stack created with Aviator. feat(expo-example): configure and add magic link login #142 fix(expo-example): add EAS local build config and fix HMR bug with secureStoreStamper #141 feat(expo-example): migrate to expo-router #131 feat(react): enable OAuth in React Native #80 feat(expo-example): add Passkey auth and test on Android #124 feat: add expo-example app #76 fix(react): race condition between initialize and isAuthorized in RN #75 feat: add fetchOptions to core and react so Origin header can be sent on RN #74 feat(react): expose stamper, storage and shouldInit params #73 ➡️ refactor(core): use atob and btoa instead of Buffer #72 main

Closes FS-1926

Removing the usage of Buffer and switching to atob and btoa + using custom code. It works but this is probably best to replace with something more robust.

[OmarBasem]

Can you add context on the PR description on why we are removing Buffer, is it to make it work with RN?

[andrascodes]

Can you add context on the PR description on why we are removing Buffer, is it to make it work with RN?

Yes. No Buffer in Hermes. Do you have a better alternative solution for this? I'd be happy to change it.

[SahilVasava]

LGTM. Correctly addresses the base64url issue by doing the explicit -→+, _→/ conversion before atob. The Uint8Array + String.fromCharCode + btoa round-trip is the standard browser-safe pattern.

The base branch was changed.

[SahilVasava]

LGTM (re-approving after force-push). atob/btoa swap is correct; base64url ↔ base64 conversion handled symmetrically.