feat: add policy pages

app/(app)/u/me/mentions/page.tsx

@@ -0,0 +1,21 @@
+"use client";
+
+/**
+ * /u/me/mentions — placeholder until the Mentions feature ships.
+ *
+ * The tab itself is wired into the profile shell already so users see
+ * an empty state instead of a 404 when they tap it.
+ */
+
+import { AtSign } from "lucide-react";
+
+export default function MentionsPage() {
+  return (
+    <div className="flex flex-col items-center text-center gap-3 py-12 text-muted-foreground">
+      <AtSign className="size-8" />
+      <p className="text-sm">
+        Mentions of you across posts will appear here. Coming soon.
+      </p>
+    </div>
+  );
+}