fix(mui): wrap app in LocalizationProvider (date-fns/cs) so DateField works at runtime

DateField (committed in 58a3bca) renders MUI X DatePicker, which requires a LocalizationProvider ancestor — without it, it throws at render time (tsc/build don't catch it). Added to MuiProvider.

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

src/admin/ui/MuiProvider.tsx

@@ -1,13 +1,18 @@
 import type { ReactNode } from "react";
 import { ThemeProvider } from "@mui/material/styles";
+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 MuiColorSchemeSync from "./MuiColorSchemeSync";
 
 export default function MuiProvider({ children }: { children: ReactNode }) {
   return (
     <ThemeProvider theme={theme} defaultMode="dark">
-      <MuiColorSchemeSync />
-      {children}
+      <LocalizationProvider dateAdapter={AdapterDateFns} adapterLocale={cs}>
+        <MuiColorSchemeSync />
+        {children}
+      </LocalizationProvider>
     </ThemeProvider>
   );
 }