…brand + logo

Reverts the W13 Google AI Studio re-skin (#397, 4c1bd1b) at the user's
request — the W11 Vercel + Linear look (warm near-black primary, off-white
canvas, square corners, blue Low badge) is restored. The TRUSCA rebrand
(W1/W2/W3 display names, logo, favicon) and W4 identifier renames are
untouched: #397 only ever changed design tokens / components / the design-
system doc, so the two concerns separate cleanly.

- index.css / button.tsx / badge.tsx / tailwind.config.ts: restored byte-
  for-byte to their pre-W13 (4c1bd1b^) state; index.css keeps only the
  TRUSCA brand name in its header comment.
- AppShell sidebar nav: pill -> rounded-md.
- DesignSystemPreview + its test: title / radius labels / section copy back
  to W11; the W1 BrandCandidates section is kept.
- design-system.md EN/KO: W11 content restored, with W4's repo-URL rename
  (trustedoss-portal -> trusca) re-applied so the brand link stays correct.
- Prototype apparatus stays deleted (not resurrected); main.tsx untouched.

Reversible both ways: the W13 design is preserved in history as 4c1bd1b, and
this rollback does not touch the (already-deleted) prototype files, so re-
adopting it later is a clean `git revert <this-commit>` (or cherry-pick the
4c1bd1b design diff). Side effect: the pending 'W13 visual-regression
baseline regen' follow-up is moot — the committed baselines are W11-era.