Make dev_server_start idempotent to prevent HMR breakage and 502s

vibn-frontend/app/[workspace]/project/[projectId]/(home)/preview/page.tsx

@@ -181,13 +181,9 @@ export default function PreviewTab() {
       const path = currentPath.startsWith("/")
         ? currentPath
         : `/${currentPath}`;
-      // Add the refreshKey as a query param so the iframe completely remounts/refetches
+      setIframeSrc(`${base}${path}`);
-      // when the user hits the manual refresh button.
-      const urlObj = new URL(`${base}${path}`);
-      urlObj.searchParams.set("_refresh", refreshKey.toString());
-      setIframeSrc(urlObj.toString());
     }
-  }, [primaryRunning?.url, currentPath, refreshKey]);
+  }, [primaryRunning?.url, currentPath]);
 
   useEffect(() => {
     if (!bridge || !iframeSrc || !iframeDomRef.current) return;

vibn-frontend/lib/dev-container.ts

@@ -873,14 +873,36 @@ export async function startDevServer(
   //    to keep the dashboard clean and prevent memory leaks.
   const existingRows = await query<{
     id: string;
-    pid: number | null;
+    project_id: string;
+    workspace: string;
+    name: string;
+    command: string;
     port: number;
+    pid: number | null;
+    preview_url: string;
+    state: "starting" | "running" | "stopped" | "failed";
+    started_at: string;
+    stopped_at: string | null;
   }>(
-    `SELECT id, pid, port FROM fs_dev_servers
+    `SELECT * FROM fs_dev_servers
       WHERE project_id = $1 AND state IN ('starting','running','failed')`,
     [opts.projectId],
   );
 
+  // IDEMPOTENCY: If the exact same command is already starting or running on the same port,
+  // do not kill it! Just return the existing record. This prevents the AI from accidentally
+  // bouncing the server and dropping the cache after every file edit, which leads to 502s.
+  const alreadyRunning = existingRows.find(
+    (r) =>
+      r.port === opts.port &&
+      r.command === opts.command &&
+      (r.state === "starting" || r.state === "running"),
+  );
+
+  if (alreadyRunning) {
+    return alreadyRunning;
+  }
+
   const killPortNodeCmd =
     `node -e '` +
     `const fs = require("fs"); ` +