feat(auth,mail): fix OAuth redirect and add Resend email provider

[Gautam25Raj]

Description

This PR resolves two core authentication and infrastructure improvements:

1. OAuth cancellation redirect fix:

   • When users cancelled third-party OAuth consent (Google or GitHub), Better Auth defaulted error redirects to https://api.veriworkly.com/?error=access_denied..., leaving users on raw API JSON.
   • Configured onAPIError: { errorURL: config.auth.studioLoginUrl } in server auth options so failed or cancelled OAuth flows redirect to Studio /login.
   • Updated Studio login page to parse OAuth errors via useSearchParams(), display a user-friendly error toast, and clean up query parameters from the address bar.
   • Removed direct window object references in SocialAuth.tsx in favor of Next.js navigation hooks and site config URLs.

2. Resend email integration and SMTP connection pooling:

   • Added native Resend provider support (AUTH_EMAIL_PROVIDER=resend) using the official resend SDK and RESEND_API_KEY.
   • Updated production runtime validation (validateAuthRuntimeConfig) to accept both resend and smtp in production, verifying RESEND_API_KEY when Resend is selected.
   • Upgraded Nodemailer SMTP transport with connection pooling (pool: true, maxConnections: 5) and socket timeouts so that SMTP relays do not hang or exhaust sockets under load.
   • Added comprehensive unit tests for Resend dispatch, missing credentials, API errors, and runtime validation.
   • Updated environment variable documentation across .env.example, .env.docker.example, compose.yaml, and docs platform.

Linked Issue

• Closes [BUG]: Cancelling OAuth login redirects to API root URL instead of Studio login page #206
• Closes [FEATURE]: Support Resend API email provider and connection-pooled SMTP #207

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📝 Documentation update
• 🧪 Tests (adding or updating tests)

Checklist

• I have read the CONTRIBUTING.md document.
• My code follows the style guidelines of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• Any dependent changes have been merged and published in downstream modules.