Add ability to block/unblock users and communities from post and comment overflow menus with optimistic UI updates and rollback on failure. Changes: - Add BlockProvider with optimistic toggle, pending state, and auto-clear on sign-out - Add blockUser/unblockUser/blockCommunity/unblockCommunity API methods - Add shared block_action_helpers with auth check, confirmation dialog, and snackbar feedback - Add block menu items to post card (user + community) and comment card (user) - Add block error message to ErrorMessage - Register BlockProvider in main.dart - Add haptic feedback on comment card tap Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>