feat(auth): auto-generate loopback client_id for local development

When clientId is any loopback URL (detected via isLoopbackUrl), the SDK
takes over completely: builds http://localhost?scope=atproto+transition:generic
&redirect_uri=<config.redirectUri> regardless of what the user passed.

- If isLoopbackUrl(clientId), always rewrite to http://localhost with params
- Scope hardcoded to 'atproto transition:generic' (AT Protocol spec requirement)
- Warn only when rewriting from non-standard loopback (e.g. 127.0.0.1:3000)
- No developmentMode flag needed — loopback URL is the signal

Author: Kzoeps