feat(mui): migrate NotFound (404) onto MUI kit

Replace legacy admin-empty-state/admin-empty-icon/admin-btn markup and motion
wrapper with a centered MUI Box + Typography (h1 variant for 404 heading,
body1 for message) and a kit Button component={RouterLink}. All text and link
target ('/') preserved verbatim.

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

src/admin/pages/NotFound.tsx

@@ -1,53 +1,28 @@
-import { Link } from "react-router-dom";
-import { motion } from "framer-motion";
+import { Link as RouterLink } from "react-router-dom";
+import Box from "@mui/material/Box";
+import Typography from "@mui/material/Typography";
+import { Button } from "../ui";
 
 export default function NotFound() {
   return (
-    <motion.div
-      className="admin-empty-state"
-      style={{ minHeight: "60vh", justifyContent: "center" }}
-      initial={{ opacity: 0, y: 12 }}
-      animate={{ opacity: 1, y: 0 }}
-      transition={{ duration: 0.25 }}
+    <Box
+      sx={{
+        display: "flex",
+        flexDirection: "column",
+        alignItems: "center",
+        justifyContent: "center",
+        minHeight: "60vh",
+        gap: 2,
+        textAlign: "center",
+      }}
     >
-      <div
-        className="admin-empty-icon"
-        style={{ width: 80, height: 80, marginBottom: "1.5rem" }}
-      >
-        <svg
-          width="36"
-          height="36"
-          viewBox="0 0 24 24"
-          fill="none"
-          stroke="currentColor"
-          strokeWidth="1.5"
-          strokeLinecap="round"
-          strokeLinejoin="round"
-        >
-          <circle cx="12" cy="12" r="10" />
-          <path d="M16 16s-1.5-2-4-2-4 2-4 2" />
-          <line x1="9" y1="9" x2="9.01" y2="9" />
-          <line x1="15" y1="9" x2="15.01" y2="9" />
-        </svg>
-      </div>
-      <h2
-        style={{
-          fontSize: "1.5rem",
-          fontWeight: 600,
-          marginBottom: "0.5rem",
-          color: "var(--text-primary)",
-        }}
-      >
+      <Typography variant="h1" sx={{ fontSize: "6rem", fontWeight: 700 }}>
         404
-      </h2>
-      <p>Stránka nebyla nalezena.</p>
-      <Link
-        to="/"
-        className="admin-btn admin-btn-primary"
-        style={{ marginTop: "0.5rem" }}
-      >
+      </Typography>
+      <Typography variant="body1">Stránka nebyla nalezena.</Typography>
+      <Button component={RouterLink} to="/" sx={{ mt: 0.5 }}>
         Zpět na Dashboard
-      </Link>
-    </motion.div>
+      </Button>
+    </Box>
   );
 }