Connect social provider with existing account

## Feature Request: Link OAuth providers to existing accounts ### Problem When a user creates an account via email/password and later tries to log in with Google or GitHub OAuth using the same email address, Plunk treats it as a new signup attempt instead of linking the OAuth provider to the existing account. With `DISABLE_SIGNUPS=true`, this results in a `New user signups are currently disabled` error. With signups enabled, the user sees `You used another form of authentication` and is redirected back to the login page without being logged in. ### Expected behavior If an OAuth login matches an existing account's email address, the OAuth provider should be linked to that account and the user should be logged in. ### Suggested implementation During the OAuth callback, check if a user with the same email already exists. If so, link the OAuth provider to the existing account instead of attempting to create a new one. This is a common pattern in platforms like GitLab, Supabase, and others. ### Environment - Plunk v0.6.0 (self-hosted, Docker) - OAuth providers: Google and GitHub - `DISABLE_SIGNUPS=true`