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/components/dashboard/DashKpiCards.tsx

@@ -109,10 +109,10 @@ function buildInvoiceKpi(invoices: InvoicesData): KpiCard {
 }
 
 const KPI_CLASS_MAP: Record<number, string> = {
-  4: "dash-kpi-4",
-  3: "dash-kpi-3",
-  2: "dash-kpi-2",
-  1: "dash-kpi-1",
+  4: "admin-kpi-4",
+  3: "admin-kpi-3",
+  2: "admin-kpi-2",
+  1: "admin-kpi-1",
 };
 
 export default function DashKpiCards({ dashData }: DashKpiCardsProps) {
@@ -121,11 +121,11 @@ export default function DashKpiCards({ dashData }: DashKpiCardsProps) {
     return null;
   }
 
-  const kpiClass = KPI_CLASS_MAP[Math.min(kpiCards.length, 4)] || "dash-kpi-4";
+  const kpiClass = KPI_CLASS_MAP[Math.min(kpiCards.length, 4)] || "admin-kpi-4";
 
   return (
     <motion.div
-      className={`dash-kpi-grid ${kpiClass}`}
+      className={`admin-kpi-grid ${kpiClass}`}
       initial={{ opacity: 0, y: 12 }}
       animate={{ opacity: 1, y: 0 }}
       transition={{ duration: 0.25, delay: 0.06 }}