feat: extended time ranges — 6-month and 1-year price history views

## Problem

The current chart is limited to 90 days. Long-term price trends (seasonal cycles, vault rotations) are not visible.

## Requested behaviour

- Additional tab options: 6 months, 1 year (if API supports the data depth)
- Investigate whether `/v1/items/{slug}/statistics` returns more than 90 days
- If not, aggregate from multiple API calls or cap at available data

## Acceptance criteria

- [ ] 6-month and 1-year tab buttons in the panel chart
- [ ] Chart renders all available data for the selected range
- [ ] Gracefully handles items with less data than the selected range
- [ ] Forecast only shown on 90d view (regression on longer ranges is less meaningful)