Apply brand identity to the frontend (ink + bronze + paper)

Replaces the default black/gray with the docs/brand palette: warm ink text on paper surfaces, bronze accent, serif headings and the Origin-mark wordmark in the header, favicon, and the 'where it came from matters' tagline. Light/dark adapt via CSS vars (ink/paper flip); bronze and paper are constant. Tailwind v4 @theme exposes bronze/paper/ink tokens and the serif stack. Buttons/inputs/cards restyled to match; brand SVGs vendored into public/.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Justin Paul <justin@jpaul.me>

frontend/app/register/page.tsx

@@ -70,9 +70,9 @@ export default function RegisterPage() {
             {loading ? "Creating…" : "Create account"}
           </Button>
         </form>
-        <p className="mt-4 text-sm text-neutral-600">
+        <p className="mt-4 text-sm text-[var(--muted)]">
           Already have an account?{" "}
-          <Link href="/login" className="underline">
+          <Link href="/login" className="text-bronze underline">
             Sign in
           </Link>
         </p>