refactor(css): migrate base.css to MUI GlobalStyles and Leaflet to styled()

Phase 2 of the "fully MUI" cleanup — eliminates two more stylesheets.

- base.css -> src/admin/GlobalStyles.tsx: the reset, typography,
  scrollbar/::selection, theme cross-fade timing and all utility
  classes now live in a single theme-aware MUI <GlobalStyles>, rendered
  inside MuiProvider so every rule resolves against theme.vars (verified
  reactive in both schemes). base.css deleted.
- attendance.css -> a styled("div") LocationMap in AttendanceLocation;
  attendance.css deleted.
- App.tsx bootstrap loader is now fully self-contained (inline spinner
  + keyframes, theme-aware background read from the data-theme attr),
  since it renders before MuiProvider/GlobalStyles mount — it no longer
  depends on the removed .admin-spinner / var(--bg-primary).

tsc -b --noEmit and npm run build clean; verified in Chrome (light +
dark): body bg, text, fonts, .text-warning/.link-accent all correct.

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

src/admin/ui/MuiProvider.tsx

@@ -4,10 +4,12 @@ import { LocalizationProvider } from "@mui/x-date-pickers/LocalizationProvider";
 import { AdapterDateFns } from "@mui/x-date-pickers/AdapterDateFns";
 import { cs } from "date-fns/locale";
 import { theme } from "../theme";
+import AppGlobalStyles from "../GlobalStyles";
 
 export default function MuiProvider({ children }: { children: ReactNode }) {
   return (
     <ThemeProvider theme={theme} defaultMode="dark">
+      <AppGlobalStyles />
       <LocalizationProvider dateAdapter={AdapterDateFns} adapterLocale={cs}>
         {children}
       </LocalizationProvider>