chore(playground): migrate dev-playground server to onPluginsReady

`server({ autoStart: false }).then(appkit => appkit.server.extend(...).start())`
is gone — `createApp` now orchestrates server start itself, with the
post-setup hook surfaced as the `onPluginsReady` config callback.

Drop `autoStart: false`, hoist the `extend` block from the trailing
`.then` chain into `onPluginsReady`, and replace the dangling promise
with `.catch(console.error)` so unhandled rejections still surface.

Tracks #280 / #291 (autoStart removal + on-plugins-ready codemod).

Author: MarioCadenas

apps/dev-playground/server/index.ts

@@ -330,7 +330,7 @@ const dashboard_pilot = createAgent({
 
 createApp({
   plugins: [
-    server({ autoStart: false }),
+    server(),
     reconnect(),
     telemetryExamples(),
     analytics({}),
@@ -385,9 +385,8 @@ createApp({
     // }),
   ],
   ...(process.env.APPKIT_E2E_TEST && { client: createMockClient() }),
-}).then((appkit) => {
-  appkit.server
-    .extend((app) => {
+  async onPluginsReady(appkit) {
+    appkit.server.extend((app) => {
       app.get("/sp", (_req, res) => {
         appkit.analytics
           .query("SELECT * FROM samples.nyctaxi.trips;")
@@ -681,9 +680,9 @@ createApp({
           res.status(404).json({ error: msg });
         }
       });
-    })
-    .start();
-});
+    });
+  },
+}).catch(console.error);
 
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  * Heuristic match for Databricks Files API's "directory not found" error.