feat(mui): mount MUI provider + sync color scheme to data-theme

Wraps AdminApp in MuiProvider (ThemeProvider v7, defaultMode=dark) and
adds MuiColorSchemeSync to mirror ThemeContext's data-theme attribute
into MUI's JS color-scheme state so useColorScheme().mode stays in sync.

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

src/admin/ui/MuiProvider.tsx

@@ -0,0 +1,13 @@
+import type { ReactNode } from "react";
+import { ThemeProvider } from "@mui/material/styles";
+import { theme } from "../theme";
+import MuiColorSchemeSync from "./MuiColorSchemeSync";
+
+export default function MuiProvider({ children }: { children: ReactNode }) {
+  return (
+    <ThemeProvider theme={theme} defaultMode="dark">
+      <MuiColorSchemeSync />
+      {children}
+    </ThemeProvider>
+  );
+}