feat: item comparison — overlay two items on the same price history chart

## Problem

Comparing price trends between two items (e.g. two prime sets to decide which to farm) requires switching back and forth between views.

## Requested behaviour

- Add a second item to compare from the panel search
- Both price history lines rendered on the same SVG chart (different colors)
- Stats row shows both items side by side
- Clear / swap comparison items

## Acceptance criteria

- [ ] "Compare with…" input or button on the item view
- [ ] Second item line overlaid on chart with distinct color
- [ ] Y-axis shared or dual (whichever is cleaner)
- [ ] Stats row shows both items' min/avg/max/median
- [ ] Remove comparison resets to single-item view