docs: fix the beforeAuthStateChanged import and the Imagen removal note

docs/auth.md

@@ -93,8 +93,8 @@ Add the cookie sync to your `app.config.ts`. AngularFire's `idToken` observable
 import { DestroyRef, PLATFORM_ID, inject, provideAppInitializer } from '@angular/core';
 import { isPlatformBrowser } from '@angular/common';
 import { takeUntilDestroyed } from '@angular/core/rxjs-interop';
-import { Auth, idToken } from '@angular/fire/auth';
-import { beforeAuthStateChanged } from 'firebase/auth';
+// on AngularFire 20.1.0 and earlier, or 21.0.0-rc.0 and rc.1, import beforeAuthStateChanged from firebase/auth (see note below)
+import { Auth, beforeAuthStateChanged, idToken } from '@angular/fire/auth';
 import cookies from 'js-cookie';
 
 // add to appConfig.providers
@@ -151,9 +151,9 @@ This cookie carries a short-lived ID token that scripts on your page can read. F
 
 If you need a session the browser cannot read, use Firebase's [session cookies](https://firebase.google.com/docs/auth/admin/manage-cookies) with the Admin SDK instead. Those cannot be handed to `initializeServerApp`, so that approach means verifying the cookie yourself and building your own server-side Auth context.
 
-#### `beforeAuthStateChanged` from `firebase/auth`
+#### On AngularFire 20.1.0 and earlier, or 21.0.0-rc.0 and rc.1
 
-One import in the code above is deliberately different from the rest of this guide. `beforeAuthStateChanged` comes from `firebase/auth` rather than `@angular/fire/auth`. In AngularFire 21.0.0-rc.1 and earlier, AngularFire's version keeps the app marked as busy until its callback first runs, and this callback only runs when someone signs in or out ([#3748](https://github.com/angular/angularfire/issues/3748)).
+On these releases, import `beforeAuthStateChanged` from `firebase/auth` instead of `@angular/fire/auth`. Otherwise it keeps the app marked as busy until its callback first runs, and this callback only runs when someone signs in or out ([#3748](https://github.com/angular/angularfire/issues/3748)). The fix shipped in AngularFire 20.1.1 and 21.0.0-rc.2.
 
 ### 3. Pass the cookie into the render
 

docs/version-21-upgrade.md

@@ -77,6 +77,8 @@ Two symbols were removed rather than renamed. The migration leaves their imports
 
 - `VertexAIOptions`: the new `AIOptions` takes a `backend` instead of a `location`, so rebuild the options by hand using the table above.
 - `getImagenModel`: Firebase shut the Imagen models down in August 2026 and removed the API in firebase 12.18 (`@firebase/ai` 2.15), so move image generation to the Gemini image models through [`getGenerativeModel`](https://firebase.google.com/docs/ai-logic/generate-images-gemini).
+  - The same release removed every other `Imagen*` export too, such as `ImagenModel`, `ImagenAspectRatio` and `ImagenGenerationConfig`.
+  - The migration does not warn about those. It warns about `getImagenModel` only in files that imported an old Vertex AI entry point, so check imports from `@angular/fire/ai` and `firebase/ai` by hand.
 
 ## Other notes
 

sample/src/app/auth/auth.component.ts

@@ -1,10 +1,10 @@
 import { Component, EnvironmentInjector, inject, makeStateKey, OnDestroy, PLATFORM_ID, runInInjectionContext, TransferState } from '@angular/core';
-import { Auth, signInAnonymously, signOut, User } from '@angular/fire/auth';
+import { Auth, beforeAuthStateChanged, signInAnonymously, signOut, User } from '@angular/fire/auth';
 import { map, startWith, switchMap, tap } from 'rxjs/operators';
 import { AsyncPipe, isPlatformBrowser, isPlatformServer } from '@angular/common';
 import cookies from 'js-cookie';
 import { from, Observable } from 'rxjs';
-import { GoogleAuthProvider, onAuthStateChanged, signInWithPopup, beforeAuthStateChanged, onIdTokenChanged } from "firebase/auth";
+import { GoogleAuthProvider, onAuthStateChanged, signInWithPopup, onIdTokenChanged } from "firebase/auth";
 import { ɵzoneWrap } from "@angular/fire";
 
 // TODO bring this to RxFire