Replace Lemmy username/password authentication with ATProto's OAuth flow for the Coves migration. This removes legacy Lemmy-specific code and simplifies the auth model for ATProto's identity system. Changes: - Add /oauth/callback route for handling OAuth redirects - Rewrite login page for handle-based OAuth initiation - Update ProfileInfo to store ATProto fields (did, sessionId, handle) - Add automatic token refresh with 401 retry in API client - Remove Lemmy-specific code (cookie migration, donation polling) - Stub legacy subscription/favorites APIs with migration TODOs - Remove inbox notification badges pending Coves API - Fix localStorage error handling and type safety Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>