fix: implement Clerk v7 custom auth flow and correct SSE parsing

[ferosai]

fix: implement Clerk v7 custom auth flow and correct SSE parsing

This commit addresses two critical issues in the frontend functionality:

1. Authentication Flow (Clerk v7 Migration):

   • Migrated the custom sign-in form to utilize the correct Clerk Next.js v7 SDK API surface (signIn.password(), signIn.mfa.sendEmailCode(), signIn.mfa.verifyEmailCode()).
   • Replaced deprecated v4/v5 methods (signIn.create, attemptFirstFactor, setActive) which were causing TypeScript build failures.
   • Implemented a complete state machine to handle Multi-Factor Authentication (MFA) and Client Trust requirements, automatically toggling the UI to prompt for an email verification code when necessary.
   • Hardened error handling by properly casting and parsing native ClerkError objects without relying on deprecated array structures.

2. API Event Stream Parser:

   • Fixed off-by-one substring index errors in the Server-Sent Events (SSE) parser (event: and data: prefixes).
   • Added multiline data concatenation logic to ensure complete payload capture when streaming chunks are split across multiple lines.
   • Added the explicit Accept: text/event-stream header for better proxy/CDN compatibility.