Error pages render within layout instead of bare nil/404

Route errors and missing pages now show a styled error message inside
the normal layout (header, nav still work) instead of bare "nil" text
or a raw "Not Found" page. AJAX errors return renderable SX error
fragments instead of "nil" strings.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/bin/sx_server.ml

@@ -1544,14 +1544,32 @@ let http_render_page env path headers =
         Printf.eprintf "[http] route error for %s: %s\n%!" path (Printexc.to_string e);
         Nil
     in
-    match route_result with
+    (* Build an error page AST that keeps the layout intact *)
-    | Nil -> None
+    let error_page_ast msg =
-    | Dict d ->
+      List [Symbol "div"; Keyword "class"; String "p-8 max-w-2xl mx-auto";
-      let is_ajax = match Hashtbl.find_opt d "is-ajax" with Some (Bool true) -> true | _ -> false in
+        List [Symbol "h2"; Keyword "class"; String "text-xl font-semibold text-rose-600 mb-4";
-      let nav_path = match Hashtbl.find_opt d "nav-path" with Some (String s) -> s | _ -> path in
+          String "Page Error"];
-      let page_ast = match Hashtbl.find_opt d "page-ast" with Some v -> v | _ -> Nil in
+        List [Symbol "p"; Keyword "class"; String "text-stone-600 mb-2"; String path];
-      if page_ast = Nil then None
+        List [Symbol "pre"; Keyword "class"; String "text-sm bg-stone-100 p-4 rounded overflow-x-auto text-stone-700";
-      else begin
+          String msg]]
+    in
+    (* Normalize route result — Nil and non-Dict become error pages *)
+    let is_ajax_req = List.exists (fun (k,_) -> String.lowercase_ascii k = "sx-request") headers in
+    let route_dict = match route_result with
+      | Dict d -> d
+      | _ ->
+        let d = Hashtbl.create 4 in
+        Hashtbl.replace d "is-ajax" (Bool is_ajax_req);
+        Hashtbl.replace d "nav-path" (String path);
+        Hashtbl.replace d "page-ast" (error_page_ast "Page not found");
+        d
+    in
+    let d = route_dict in
+    let is_ajax = match Hashtbl.find_opt d "is-ajax" with Some (Bool true) -> true | _ -> false in
+    let nav_path = match Hashtbl.find_opt d "nav-path" with Some (String s) -> s | _ -> path in
+    let page_ast = match Hashtbl.find_opt d "page-ast" with Some v -> v | _ -> Nil in
+    let page_ast = if page_ast = Nil then error_page_ast "Page returned empty content" else page_ast in
+      begin
         let wrapped = List [Symbol "~layouts/doc"; Keyword "path"; String nav_path; page_ast] in
         if is_ajax then begin
           (* AJAX: return SX wire format (aser output) with text/sx content type *)
@@ -1618,9 +1636,6 @@ let http_render_page env path headers =
           Some html
         end
       end
-    | _ ->
-      Printf.eprintf "[http] unexpected handler result for %s\n%!" path;
-      None
 
 (* ====================================================================== *)
 (* Static file serving + file hashing                                      *)
@@ -2409,10 +2424,13 @@ let http_mode port =
                   | Some body ->
                     let resp = http_response ~content_type:"text/sx; charset=utf-8" body in
                     Hashtbl.replace response_cache cache_key resp; resp
-                  | None -> http_response ~status:404 "nil"
+                  | None -> http_response ~status:404
+                    "(div :class \"p-8\" (h2 :class \"text-rose-600 font-semibold\" \"Page not found\") (p :class \"text-stone-500\" \"No route matched this path\"))"
                 with e ->
                   Printf.eprintf "[ajax] Error for %s: %s\n%!" path (Printexc.to_string e);
-                  http_response ~status:500 "nil"
+                  http_response ~status:500
+                    (Printf.sprintf "(div :class \"p-8\" (h2 :class \"text-rose-600 font-semibold\" \"Render Error\") (pre :class \"text-sm bg-stone-100 p-4 rounded\" \"%s\"))"
+                      (escape_sx_string (Printexc.to_string e)))
               in
               write_response fd response; true
             end else begin