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/buttons.css

@@ -0,0 +1,130 @@
+/* ============================================================================
+   Buttons
+   ============================================================================ */
+
+.admin-btn {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  gap: 0.5rem;
+  padding: 8px 14px;
+  border: none;
+  border-radius: var(--border-radius-sm);
+  font-size: 13px;
+  font-weight: 550;
+  font-family: inherit;
+  cursor: pointer;
+  transition: var(--transition);
+  white-space: nowrap;
+}
+
+.admin-btn:disabled {
+  opacity: 0.7;
+  cursor: not-allowed;
+}
+
+/* Prevent buttons from growing when text changes to spinner + loading text */
+.admin-modal-footer .admin-btn {
+  min-width: 100px;
+}
+.admin-modal-footer .admin-btn .admin-spinner {
+  margin: -2px 0;
+}
+
+.admin-btn-sm {
+  padding: 6px 11px;
+  font-size: 12px;
+}
+
+.admin-btn-primary {
+  background: var(--accent-color);
+  color: #fff;
+}
+
+.admin-btn-primary:hover:not(:disabled) {
+  background: var(--accent-hover);
+  transform: translateY(-1px);
+  box-shadow: 0 4px 12px rgba(214, 48, 49, 0.3);
+}
+
+.admin-btn .admin-spinner {
+  width: 16px;
+  height: 16px;
+  border-width: 2px;
+}
+
+.admin-btn-primary .admin-spinner {
+  border-color: rgba(255, 255, 255, 0.3);
+  border-top-color: #fff;
+}
+
+.admin-btn-secondary .admin-spinner {
+  border-color: rgba(var(--text-secondary-rgb, 107, 114, 128), 0.3);
+  border-top-color: var(--text-secondary);
+}
+
+.admin-btn-secondary {
+  background: var(--bg-tertiary);
+  border: 1px solid var(--border-color);
+  color: var(--text-secondary);
+}
+
+.admin-btn-secondary:hover:not(:disabled) {
+  background: var(--bg-secondary);
+  border-color: var(--border-color-hover);
+  color: var(--text-primary);
+}
+
+.admin-btn-icon {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  padding: 0;
+  width: 32px;
+  height: 32px;
+  background: transparent;
+  border: none;
+  color: var(--text-secondary);
+  border-radius: var(--border-radius-sm);
+  cursor: pointer;
+  transition: var(--transition);
+}
+
+.admin-btn-icon:hover {
+  background: var(--bg-tertiary);
+  color: var(--text-primary);
+}
+
+.admin-btn-icon.accent {
+  color: var(--info);
+}
+
+.admin-btn-icon.accent:hover {
+  background: color-mix(in srgb, var(--info) 15%, transparent);
+  color: var(--info);
+}
+
+.admin-btn-icon.danger {
+  color: var(--danger);
+}
+
+.admin-btn-icon.danger:hover {
+  background: var(--danger-light);
+}
+
+/* Touch targets - min 44px on mobile */
+@media (max-width: 768px) {
+  .admin-btn {
+    min-height: 44px;
+    padding: 10px 16px;
+  }
+
+  .admin-btn-sm {
+    min-height: 36px;
+  }
+
+  .admin-btn-icon {
+    min-width: 44px;
+    min-height: 44px;
+  }
+}