Write the spec

The written answer to what we're building. You may know it as a PRD.

spec.md
User story
As a player, I want to see where an item comes from, so that I know where to find it.
Decisions
Redesign the layout first, then add the item source. The inventory list gets a light cleanup in the same pass.
Tested
The new layout renders and the item source shows.
Out of scope
Bulk edits and CSV export.
Writing this list is what keeps the feature small.