Add weekly-grouped versions of existing monthly summary visuals #116
Description
Most current summary visuals aggregate data by month, which makes it hard to spot shorter-term training patterns and changes in load.
Introduce a parallel set of weekly-grouped summary visuals alongside the
existing monthly versions. These should:
- Aggregate metrics by calendar week (Monday–Sunday)
- Mirror the intent of the current monthly visuals rather than invent new ones
- Make short-term trends and changes in training load more visible
- Be rendered as clean PNGs suitable for README embedding
These charts complement the monthly versions, rather than replacing them.