refactor admin CSS: replace utility classes with semantic styles

Replace Tailwind utility soup across admin templates with semantic
CSS classes. Add layout primitives (stack, row, cluster, grid),
extract JS transition helpers into transitions.css, and refactor
core_components, layouts, settings, newsletter, order_show, providers,
and theme editor templates.

Utility occurrences reduced from 290+ to 127 across admin files.
All 1679 tests pass.

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

assets/css/admin/layout.css

@@ -0,0 +1,36 @@
+/* Admin layout primitives — composable building blocks mirroring shop/layout.css.
+   Each primitive does one layout job. Combine freely via CSS custom properties. */
+
+@layer admin {
+  /* Vertical stack with consistent gap */
+  .admin-stack {
+    display: flex;
+    flex-direction: column;
+    gap: var(--admin-stack-gap, 1rem);
+  }
+
+  /* Horizontal flex row, no wrap — toolbars, inline groups, header bars */
+  .admin-row {
+    display: flex;
+    align-items: center;
+    gap: var(--admin-row-gap, 0.5rem);
+  }
+
+  /* Horizontal flex-wrap cluster — tags, badges, button groups */
+  .admin-cluster {
+    display: flex;
+    flex-wrap: wrap;
+    gap: var(--admin-cluster-gap, 0.5rem);
+    align-items: center;
+  }
+
+  /* Intrinsic responsive grid — cards, media thumbnails */
+  .admin-grid {
+    display: grid;
+    grid-template-columns: repeat(
+      auto-fill,
+      minmax(min(var(--admin-grid-min, 16rem), 100%), 1fr)
+    );
+    gap: var(--admin-grid-gap, 1rem);
+  }
+}