refactor: split admin.css monolith, standardize CSS architecture

- Split admin.css (3228 lines) into 12 focused files: variables, base,
  forms, buttons, layout, components, tables, skeleton, datepicker,
  filemanager, pagination, responsive
- Extracted shared styles from offers.css and dashboard.css into
  components.css and forms.css (offers-* → admin-* prefix)
- Standardized naming: dash-kpi-* → admin-kpi-*, session-* → dash-session-*,
  rich-editor → admin-rich-editor
- Deleted duplicate offers-tabs (using admin-tabs everywhere)
- Deduplicated DatePicker and FileManager CSS (~360 lines removed)
- Added 16 utility classes to base.css (font sizes, widths, gaps, margins)
- Deleted empty admin.css

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/admin/skeleton.css

@@ -0,0 +1,72 @@
+/* ============================================================================
+   Skeleton Loading
+   ============================================================================ */
+
+.admin-skeleton {
+  display: flex;
+  flex-direction: column;
+  gap: 1rem;
+  padding: 1.5rem;
+  opacity: 0;
+  animation: skeleton-fade-in 0.15s ease 0.08s forwards;
+}
+
+@keyframes skeleton-fade-in {
+  to {
+    opacity: 1;
+  }
+}
+
+.admin-skeleton-row {
+  display: flex;
+  gap: 1rem;
+  align-items: center;
+}
+
+.admin-skeleton-line {
+  height: 14px;
+  border-radius: 6px;
+  background: linear-gradient(
+    90deg,
+    var(--bg-tertiary) 25%,
+    var(--border-color) 50%,
+    var(--bg-tertiary) 75%
+  );
+  background-size: 200% 100%;
+  animation: shimmer 1.2s ease-in-out infinite;
+}
+
+.admin-skeleton-line.w-full {
+  width: 100%;
+}
+.admin-skeleton-line.w-3\/4 {
+  width: 75%;
+}
+.admin-skeleton-line.w-1\/2 {
+  width: 50%;
+}
+.admin-skeleton-line.w-1\/3 {
+  width: 33%;
+}
+.admin-skeleton-line.w-1\/4 {
+  width: 25%;
+}
+.admin-skeleton-line.h-8 {
+  height: 32px;
+}
+.admin-skeleton-line.h-10 {
+  height: 40px;
+}
+.admin-skeleton-line.circle {
+  width: 40px;
+  height: 40px;
+  border-radius: 50%;
+  flex-shrink: 0;
+}
+
+/* Skeleton loading on mobile */
+@media (max-width: 640px) {
+  .admin-skeleton {
+    border-radius: 4px;
+  }
+}