feat: Stage 3 — application shell and navigation

frontend/src/pages/JailDetailPage.tsx

@@ -0,0 +1,25 @@
+/**
+ * Jail detail placeholder page — full implementation in Stage 6.
+ */
+
+import { Text, makeStyles, tokens } from "@fluentui/react-components";
+import { useParams } from "react-router-dom";
+
+const useStyles = makeStyles({
+  root: { padding: tokens.spacingVerticalXXL },
+});
+
+export function JailDetailPage(): JSX.Element {
+  const styles = useStyles();
+  const { name } = useParams<{ name: string }>();
+  return (
+    <div className={styles.root}>
+      <Text as="h1" size={700} weight="semibold">
+        Jail: {name}
+      </Text>
+      <Text as="p" size={300}>
+        Jail detail view will be implemented in Stage 6.
+      </Text>
+    </div>
+  );
+}