feat(trade+items): add resistance and damage element swap evaluation

Port of feature/resistance-swap onto current origin/dev.
Adapted to upstream PathOfBuildingCommunity#9691 changes (removed enchantInSort, new popup
layout, Watcher's Eye support, updated sort algorithm).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mcagnion