Add a complete XRPC transport layer and typed API client for the Coves backend, replacing the need for Lemmy/PiFed adapters for Coves-native endpoints. Changes: - Add XrpcClient with query (GET) and procedure (POST) methods that parse structured XRPC error bodies into typed XrpcError instances - Add CovesClient with typed methods for all 18 Coves API endpoints (feed, comments, votes, actor, community, posts) - Add covesCustomFetch that returns raw Response (unlike customFetch) so XrpcClient can parse XRPC error bodies before throwing - Add coves() factory function integrated with profile/proxy routing - Strengthen types: add PostRecord, CommentRecord, CommunityVisibility union type, and replace generic string types with literal unions - Add 46 tests covering XRPC transport, client delegation, and type validation utilities Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>