replace --color-* with --t-* tokens, delete bridge and .preview-frame

Phase 4: admin components and utilities now reference --t-* theme
tokens directly. Status colour tokens added to theme-semantic.css.
Bridge file (admin/themes.css) deleted.

Phase 5: removed duplicated .preview-frame CSS block (~160 lines).
Admin components and icons wrapped in @layer admin. Layer order
updated in admin_root to include admin layer.

Phase 6: added prefers-reduced-motion support (zeroes all durations
and disables animations). Migrated physical properties to logical
equivalents (text-align start/end, margin-inline, padding-inline,
inset-inline-end) across shop and admin CSS.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

assets/css/admin/icons.css

@@ -1,5 +1,7 @@
 /* Generated by mix generate_admin_icons — do not edit by hand */
 
+@layer admin {
+
 .hero-arrow-left-mini {
   --hero-arrow-left-mini: url('data:image/svg+xml;utf8,<svg%20xmlns="http://www.w3.org/2000/svg"%20viewBox="0%200%2020%2020"%20fill="currentColor"%20aria-hidden="true"%20data-slot="icon">%20%20<path%20fill-rule="evenodd"%20d="M17%2010a.75.75%200%200%201-.75.75H5.612l4.158%203.96a.75.75%200%201%201-1.04%201.08l-5.5-5.25a.75.75%200%200%201%200-1.08l5.5-5.25a.75.75%200%201%201%201.04%201.08L5.612%209.25H16.25A.75.75%200%200%201%2017%2010Z"%20clip-rule="evenodd"/></svg>');
   -webkit-mask: var(--hero-arrow-left-mini);
@@ -623,3 +625,5 @@
   width: 1.5rem;
   height: 1.5rem;
 }
+
+} /* @layer admin */