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/Dashboard.tsx

@@ -338,7 +338,7 @@ export default function Dashboard() {
       {/* Skeleton loading */}
       {dashLoading && (
         <div className="admin-skeleton" style={{ padding: 0, gap: "1.25rem" }}>
-          <div className="dash-kpi-grid dash-kpi-4">
+          <div className="admin-kpi-grid admin-kpi-4">
             {[0, 1, 2, 3].map((i) => (
               <div
                 key={i}