- Settings: add unit_system key (metric | imperial, default metric)
- Recipe LLM prompts: inject unit instruction into L3 and L4 prompts
so generated recipes use the user's preferred units throughout
- Frontend: new utils/units.ts converter (mirrors Python units.py)
- Inventory list: display quantities converted to preferred units
- Settings view: metric/imperial toggle with save button
- Settings store: load/save unit_system alongside cooking_equipment
Closes#81
