When 'Show Memory Stats' debug mode is enabled, the block footer
now displays 'Request tokens: N' (or 'Request tokens: --') instead
of the old grid/flat storage size breakdown.
- Add last_block_token_usage_by_model to AIConversation, tracking
token usage accumulated since the most recent user-initiated request
- Add last_block_total_tokens() accessor summing across all models
- BlockListElement reads this value per block and renders it in the
debug footer text element
Ryan Ward
ed53aa99eb