shadcn cleanup: retire theme.css + xore/theme token handoff

[Xore]

Part of #3201 (shadcn/ui migration epic).

Retire legacy theme.css + xore/theme token handoff

Steps:

1. After #shadcn-9: grep every theme.css class — remaining references must be zero (or only third-party lib requirements documented inline). Delete dead blocks; end state is an empty file or a stub comment pointing at src/index.css.
2. Delete the temporary /dev/gallery route.
3. xore/theme repo: add the final token file (the shadcn token layer as consumed by the dashboard) + short doc; PR to xore/theme, merge on user go (standing rule).
4. Dashboard README/docs: update styling docs to the shadcn/Tailwind v4 architecture.

Gate: grep clean, build clean, epic #3201 closeout checklist in #shadcn-9 satisfied.