Phase 1: Add client-side ATProto fetch library and shared types
New src/lib/atproto-client.ts provides unauthenticated client-side functions
for resolving DIDs to handles, fetching game records from PDS, and fetching
moves/passes from Constellation backlinks. Includes PDS fallback.
New src/lib/types.ts defines shared interfaces for ATProto record shapes.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>