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/transitions.css

@@ -0,0 +1,80 @@
+/* Transition and animation utilities for admin pages.
+   These are required by Phoenix JS.show/JS.hide in core_components.ex
+   and by various admin UI interactions (spinners, hover effects, etc.).
+
+   Placed in @layer admin so they integrate with the cascade. */
+
+@layer admin {
+
+/* ── Transition properties ── */
+
+.transition {
+  transition-property: color, background-color, border-color, text-decoration-color,
+    fill, stroke, opacity, box-shadow, transform, filter, backdrop-filter;
+  transition-timing-function: cubic-bezier(0.4, 0, 0.2, 1);
+  transition-duration: 150ms;
+}
+
+.transition-all {
+  transition-property: all;
+  transition-timing-function: cubic-bezier(0.4, 0, 0.2, 1);
+  transition-duration: 150ms;
+}
+
+.transition-colors {
+  transition-property: color, background-color, border-color, text-decoration-color, fill, stroke;
+  transition-timing-function: cubic-bezier(0.4, 0, 0.2, 1);
+  transition-duration: 150ms;
+}
+
+.transition-transform {
+  transition-property: transform;
+  transition-timing-function: cubic-bezier(0.4, 0, 0.2, 1);
+  transition-duration: 150ms;
+}
+
+.transition-\[left\] {
+  transition-property: left;
+  transition-timing-function: cubic-bezier(0.4, 0, 0.2, 1);
+  transition-duration: 150ms;
+}
+
+/* ── Duration overrides ── */
+
+.duration-200 { transition-duration: 200ms; }
+.duration-300 { transition-duration: 300ms; }
+
+/* ── Easing overrides ── */
+
+.ease-in { transition-timing-function: cubic-bezier(0.4, 0, 1, 1); }
+.ease-out { transition-timing-function: cubic-bezier(0, 0, 0.2, 1); }
+
+/* ── Opacity states (JS.show / JS.hide) ── */
+
+.opacity-0 { opacity: 0; }
+.opacity-100 { opacity: 1; }
+
+/* ── Transform states (JS.show / JS.hide) ── */
+
+.translate-y-4 { translate: 0 1rem; }
+.translate-y-0 { translate: 0 0; }
+.scale-95 { scale: 0.95; }
+.scale-100 { scale: 1; }
+
+@media (min-width: 640px) {
+  .sm\:translate-y-0 { translate: 0 0; }
+  .sm\:scale-95 { scale: 0.95; }
+  .sm\:scale-100 { scale: 1; }
+}
+
+/* ── Spin animation ── */
+
+@keyframes spin { to { transform: rotate(360deg); } }
+
+.animate-spin { animation: spin 1s linear infinite; }
+
+@media (prefers-reduced-motion: no-preference) {
+  .motion-safe\:animate-spin { animation: spin 1s linear infinite; }
+}
+
+} /* @layer admin */