Enables authenticated writes to user PDS by adding DPoP (Demonstrating Proof-of-Possession) support to OAuthSession.fetchHandler(). Changes: - Replace http.Client with Dio + DPoP interceptor - Automatically add DPoP headers with JWT proofs and token binding (ath) - Handle nonce management and automatic retry on nonce errors - Add proper DioException handling for network/timeout errors - Remove deprecated _makeRequest method and _httpClient field This unblocks voting functionality by matching the Expo oauth-client-expo DPoP implementation, allowing the Flutter app to write records to the PDS. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>