fix: align Better Auth dependencies for BTST v3 RC

[olliethedev]

Summary

• release @btst/better-auth-ui@2.0.0-rc.1 against @btst/stack@3.0.0-rc.1 and @btst/yar@1.3.0
• migrate auth, account, and organization route registration to the BTST v3 defineRoute / defineRoutes contract
• preserve lazy page loading and the inferred plugin route/override types
• expose one exact Better Auth 1.6.16 dependency cohort through peers, including API key, passkey, utils, fetch, and better-call
• remove the conflicting nested better-call@2.0.2 and auth helper dependencies
• publish prereleases with the npm next tag and run CI/release jobs on Node 22
• add route-contract and manifest regression coverage

Refs better-stack-ai/better-stack#163.

Validation

• pnpm build
• pnpm test — 49 tests passed
• pnpm lint — passed (existing warnings remain in untouched files)
• packed tarball installed in a representative BTST v3 consumer; pnpm peers check reports no issues