refactor(css): scope Quill editor into styled() and delete offers.css

Phase 3 of the "fully MUI" cleanup.

- New src/admin/ui/RichText.tsx exports two theme-aware styled() wrappers:
  RichEditorRoot (the .admin-rich-editor Quill overrides) and RichTextView
  (the .admin-rich-text-view read-only output). All colors resolve via
  theme.vars; picker/tooltip shadows use theme.applyStyles for the
  light/dark difference; the active-button tint uses the primary channel.
- Dropped the ~170 dead font/size picker rules from the old stylesheet —
  the editor toolbar exposes neither picker. Kept the .ql-font-* content
  classes (shared by editor + view) so legacy stored HTML still renders.
- RichEditor uses <RichEditorRoot> (min-height still via --re-min-height);
  InvoiceDetail + OrderDetail use <RichTextView> for the sanitized output.
- offers.css deleted; its import removed from AdminApp.

tsc -b --noEmit and npm run build clean. Verified the editor in Chrome
(light + dark): toolbar, editor surface, text, icon strokes and min-height
all correct and theme-reactive.

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

src/admin/AdminApp.tsx

@@ -12,7 +12,6 @@ import { LoadingState } from "./ui";
 import Login from "./pages/Login";
 import Dashboard from "./pages/Dashboard";
 import "./variables.css";
-import "./offers.css";
 
 const Users = lazy(() => import("./pages/Users"));
 const Attendance = lazy(() => import("./pages/Attendance"));