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/pages/ReceivedInvoices.tsx

@@ -549,7 +549,7 @@ export default function ReceivedInvoices({
   const renderKpi = () => {
     if (!hasLoadedOnce.current && statsLoading) {
       return (
-        <div className="dash-kpi-grid dash-kpi-4 mb-6">
+        <div className="admin-kpi-grid admin-kpi-4 mb-6">
           {[0, 1, 2, 3].map((i) => (
             <div key={i} className="admin-stat-card">
               <div
@@ -586,7 +586,7 @@ export default function ReceivedInvoices({
         >
           <motion.div
             key={slideKey}
-            className="dash-kpi-grid dash-kpi-4"
+            className="admin-kpi-grid admin-kpi-4"
             custom={slideDirection.current}
             variants={{
               enter: (dir: number) => ({