fix: replace undefined __dirname with DATA_DIR in ESM server

__dirname was removed during utils.ts extraction but still referenced
in staticDir computation, causing a ReferenceError crash on startup.

Made-with: Cursor

backend/src/server.ts

@@ -351,8 +351,8 @@ app.use("/published", express.static(join(DATA_DIR, "published")));
 
 const staticDir =
   process.env.NODE_ENV === "production"
-    ? join(__dirname, "..", "frontend-dist")
-    : join(__dirname, "..", "..", "frontend", "dist");
+    ? join(DATA_DIR, "..", "frontend-dist")
+    : join(DATA_DIR, "..", "..", "frontend", "dist");
 
 function getPublishedPath(docName: string): string {
   return join(DATA_DIR, "published", docName, "index.html");