Auto-login user on successful registration via secure auth cookie

[KGFCH2]

Summary

After account registration, automatically authenticate the user by storing a secure server-issued auth cookie.

Details

• Improves UX by removing extra login step after signup
• Uses secure cookie handling for authentication

Acceptance Criteria

• New users are logged in immediately after registration
• Authentication uses secure server cookie
• No client-side auth token storage is introduced

[HarshYadav152]

@KGFCH2 This is an intentional feature so user register first and then login by entering credentials.