Add complete checkpoint restoration system allowing users to rewind conversation and room state to any previous point via double-Esc. Backend (TypeScript): - Integrate SDK file checkpointing with enableFileCheckpointing - Forward checkpoint UUIDs via CheckpointCreatedMessage IPC - Handle RestoreMessage with session forking and file rewind Frontend (Rust): - Log CheckpointCreated entries with room state snapshots - Create checkpoint picker UI (checkpoint_picker.rs) - Implement restore_to_checkpoint with session forking - Add debug server endpoints for automated testing Checkpoints are created automatically after each assistant response. Double-Esc opens picker showing checkpoints newest-first. Restoring creates new session via SDK forkSession and rewinds both chat history and room state (notes, images) from JSON snapshot. Fixes #1 in ISSUES.md