Update 02_ecommerce notebook: add HF login, memory-free cell, subsample option for <64GB RAM machines

Add 02_ecommerce_pretrain.ipynb — REES46 e-commerce pre-training with sequential entropy check, wandb, push to hub

Fix notebook: total_mem → total_memory, add hub_model_id push, add wandb logging support

Add 01_finance_pretrain.ipynb — Phase 3.1 notebook for pre-training on 5M Nigerian financial transactions