refactor(css): delete dead pre-MUI CSS and eliminate plan.css

Phase 1 of the "fully MUI" CSS cleanup.

- base.css 429->317: drop dead .admin-sidebar/header/card/modal,
  .admin-drag-handle/-kbd/-mono/-error-stack/-loading and the unused
  float/pulse keyframes. Keep reset, typography, scrollbar,
  ::view-transition cross-fade, the live .admin-spinner (App.tsx
  pre-React loader) and all utility classes.
- attendance.css 456->24: keep only the Leaflet .attendance-location-map
  rules; the old hand-rolled Attendance page is MUI now.
- offers.css 642->417: keep only the Quill (.admin-rich-editor/.ql-*)
  and .admin-rich-text-view rules; drop the dead offers table / filter /
  lang-badge / row-state rules.
- plan.css deleted: the grid already lives in PlanGrid's styled()
  wrapper; the last consumer (.plan-cat-color) is now an sx-styled
  Box input in PlanCategoriesModal.

tsc -b --noEmit and npm run build both clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/AdminApp.tsx

@@ -14,7 +14,6 @@ import Dashboard from "./pages/Dashboard";
 import "./variables.css";
 import "./base.css";
 import "./attendance.css";
-import "./plan.css";
 import "./offers.css";
 
 const Users = lazy(() => import("./pages/Users"));