chore: install the @backstage/plugin-app by default now

Signed-off-by: blam <ben@blam.sh>

.changeset/fresh-apes-dress.md

@@ -0,0 +1,8 @@
+---
+'@backstage/frontend-plugin-api': patch
+'@backstage/frontend-test-utils': patch
+'@backstage/frontend-app-api': patch
+'@backstage/plugin-app': minor
+---
+
+Introduce the `@backstage/plugin-app` package to hold all of the built-in extensions for easy consumption and overriding.

packages/frontend-app-api/package.json

@@ -38,6 +38,7 @@
     "@backstage/core-plugin-api": "workspace:^",
     "@backstage/errors": "workspace:^",
     "@backstage/frontend-plugin-api": "workspace:^",
+    "@backstage/plugin-app": "workspace:^",
     "@backstage/theme": "workspace:^",
     "@backstage/types": "workspace:^",
     "@backstage/version-bridge": "workspace:^",
@@ -48,7 +49,6 @@
   },
   "devDependencies": {
     "@backstage/cli": "workspace:^",
-    "@backstage/plugin-app": "workspace:^",
     "@backstage/test-utils": "workspace:^",
     "@testing-library/jest-dom": "^6.0.0",
     "@testing-library/react": "^15.0.0"

packages/frontend-app-api/src/wiring/createApp.test.tsx

@@ -359,4 +359,27 @@ describe('createApp', () => {
 
     expect(screen.queryByText('Custom loading message')).toBeNull();
   });
+
+  it('should allow overriding the app plugin', async () => {
+    const app = createApp({
+      configLoader: () => new Promise(() => {}),
+      features: [
+        appPlugin.withOverrides({
+          extensions: [
+            appPlugin.getExtension('app/root').override({
+              factory: () => [
+                coreExtensionData.reactElement(
+                  <div>Custom app root element</div>,
+                ),
+              ],
+            }),
+          ],
+        }),
+      ],
+    });
+
+    await renderWithEffects(app.createRoot());
+
+    expect(screen.queryByText('Custom app root element')).toBeNull();
+  });
 });

packages/frontend-app-api/src/wiring/createApp.tsx

@@ -76,6 +76,7 @@ import { ApiRegistry } from '../../../core-app-api/src/apis/system/ApiRegistry';
 // eslint-disable-next-line @backstage/no-relative-monorepo-imports
 import { AppIdentityProxy } from '../../../core-app-api/src/apis/implementations/IdentityApi/AppIdentityProxy';
 import { BackstageRouteObject } from '../routing/types';
+import appPlugin from '@backstage/plugin-app';
 
 function deduplicateFeatures(
   allFeatures: FrontendFeature[],
@@ -269,10 +270,12 @@ export function createSpecializedApp(options?: {
   bindRoutes?(context: { bind: CreateAppRouteBinder }): void;
 }): { createRoot(): JSX.Element } {
   const {
-    features: duplicatedFeatures = [],
+    features: featuresWithoutApp = [],
     config = new ConfigReader({}, 'empty-config'),
   } = options ?? {};
 
+  const duplicatedFeatures = [appPlugin, ...featuresWithoutApp];
+
   const features = deduplicateFeatures(duplicatedFeatures);
 
   const tree = resolveAppTree(