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/AdminApp.tsx

@@ -7,7 +7,18 @@ import AdminLayout from "./components/AdminLayout";
 import AlertContainer from "./components/AlertContainer";
 import Login from "./pages/Login";
 import Dashboard from "./pages/Dashboard";
-import "./admin.css";
+import "./variables.css";
+import "./base.css";
+import "./forms.css";
+import "./buttons.css";
+import "./layout.css";
+import "./components.css";
+import "./tables.css";
+import "./skeleton.css";
+import "./datepicker.css";
+import "./filemanager.css";
+import "./pagination.css";
+import "./responsive.css";
 import "./login.css";
 import "./dashboard.css";
 import "./attendance.css";