style: make all panels equal-sized

src/app.config.ts

@@ -54,7 +54,9 @@ export const AppConfig = {
       url: getHostname(),
     },
   },
-  panelWidths: {},
+  panelWidths: {
+    datasource_selector: 400,
+  },
   // allowAddExternalDatasets: false,
   panelsEnabled: {
     composition_browser: false,