Design the high-level architecture for a real-time collaborative document editing platform similar to Google Docs. Your response must address the following points: 1) Conflict resolution strategy: Compare CRDTs (Conflict-free Replicated Data Types) vs. Operational Transformation (OT) and justify your choice for a mobile-first environment with unstable connections. 2) Consistency model: Define how you will manage strong vs. eventual consistency across different data centers. 3) Storage layer: Propose a database schema that supports versioning and efficient diff retrieval. 4) Scalability: Detail how the architecture handles millions of concurrent active documents, including load balancing and sharding strategies. 5) Offline support: Explain the queuing mechanism and sync protocol for when a user goes offline and comes back online.