docs: Update Firebase authentication setup and configuration documentation

docs/06-concepts/11-authentication/04-providers/06-firebase/02-customizations.md

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+# Customizations
+
+This page covers additional configuration options for the Firebase identity provider beyond the basic setup.
+
+## Loading credentials with FirebaseIdpConfig
+
+The [setup guide](./setup) uses `FirebaseIdpConfigFromPasswords`, which loads the service account key from `passwords.yaml` for you. When you need to load credentials from a different source (a file path, a secrets manager, or just a project ID), use `FirebaseIdpConfig` directly and pass a `FirebaseServiceAccountCredentials` instance.
+
+`FirebaseServiceAccountCredentials` provides four constructors. These are the only supported ways to construct it:
+
+**From a JSON string** (use this when reading the JSON from a secrets manager or environment variable):
+
+```dart
+final firebaseIdpConfig = FirebaseIdpConfig(
+  credentials: FirebaseServiceAccountCredentials.fromJsonString(
+    pod.getPassword('firebaseServiceAccountKey')!,
+  ),
+);
+```
+
+**From a JSON file** (useful for local development or when secrets are mounted as files):
+
+```dart
+import 'dart:io';
+
+final firebaseIdpConfig = FirebaseIdpConfig(
+  credentials: FirebaseServiceAccountCredentials.fromJsonFile(
+    File('config/firebase_service_account_key.json'),
+  ),
+);
+```
+
+**From a JSON map** (useful when credentials are assembled programmatically):
+
+```dart
+final firebaseIdpConfig = FirebaseIdpConfig(
+  credentials: FirebaseServiceAccountCredentials.fromJson({
+    'type': 'service_account',
+    'project_id': 'your-project-id',
+    'private_key_id': '...',
+    'private_key': '-----BEGIN PRIVATE KEY-----\n...\n-----END PRIVATE KEY-----\n',
+    'client_email': 'firebase-adminsdk-xxxxx@your-project-id.iam.gserviceaccount.com',
+    'client_id': '...',
+    'auth_uri': 'https://accounts.google.com/o/oauth2/auth',
+    'token_uri': 'https://oauth2.googleapis.com/token',
+  }),
+);
+```
+
+**Project ID only** (only token verification, no admin operations like deleting Firebase accounts):
+
+```dart
+final firebaseIdpConfig = FirebaseIdpConfig(
+  credentials: const FirebaseServiceAccountCredentials(
+    projectId: 'your-project-id',
+  ),
+);
+```
+
+:::note
+Only `projectId` is required to verify Firebase ID tokens. The full service account JSON is only needed if you also use the [admin operations](./admin-operations) on the server.
+:::
+
+## Custom account validation
+
+You can customize the validation for Firebase account details before allowing sign-in. By default, the validation requires the email to be verified when present (phone-only authentication is allowed without an email).
+
+To customize validation, provide your own `firebaseAccountDetailsValidation` function:
+
+```dart
+final firebaseIdpConfig = FirebaseIdpConfigFromPasswords(
+  firebaseAccountDetailsValidation: (accountDetails) {
+    // Require verified email (even for phone auth)
+    if (accountDetails.verifiedEmail != true) {
+      throw Exception('Email must be verified');
+    }
+
+    // Restrict to specific email domain
+    if (accountDetails.email != null &&
+        !accountDetails.email!.endsWith('@example.com')) {
+      throw Exception('Only @example.com emails allowed');
+    }
+  },
+);
+```
+
+### FirebaseAccountDetails properties
+
+The `firebaseAccountDetailsValidation` callback receives a `FirebaseAccountDetails` record with the following properties:
+
+- `userIdentifier` (`String`): Firebase UID.
+- `email` (`String?`): Email address, or `null` for phone-only sign-in.
+- `fullName` (`String?`): Display name from Firebase.
+- `image` (`Uri?`): Profile image URL.
+- `verifiedEmail` (`bool?`): Whether the email is verified.
+- `phone` (`String?`): Phone number, only populated for phone authentication.
+
+Which properties are populated depends on the Firebase sign-in method the user chose. For example, `phone` is only populated for phone authentication, and `email` may be `null` if the user signed in with phone only.
+
+## Reacting to auth user creation
+
+[`onBeforeAuthUserCreated`](https://pub.dev/documentation/serverpod_auth_idp_server/latest/core/AuthUsersConfig/onBeforeAuthUserCreated.html) and [`onAfterAuthUserCreated`](https://pub.dev/documentation/serverpod_auth_idp_server/latest/core/AuthUsersConfig/onAfterAuthUserCreated.html) are global callbacks on `AuthUsersConfig`. They fire for every identity provider, not just Firebase. See [Working with users](../../working-with-users#reacting-to-the-user-created-event) for full details.
+
+The example below uses Firebase phone numbers as the trigger for assigning a `phone-verified` scope at sign-up, and persists the Firebase UID for later admin lookups:
+
+```dart
+pod.initializeAuthServices(
+  tokenManagerBuilders: [
+    JwtConfigFromPasswords(),
+  ],
+  identityProviderBuilders: [
+    FirebaseIdpConfigFromPasswords(),
+  ],
+  authUsersConfig: AuthUsersConfig(
+    onBeforeAuthUserCreated: (
+      session,
+      scopes,
+      blocked, {
+      required transaction,
+    }) {
+      return (
+        scopes: {...scopes, Scope('user')},
+        blocked: blocked,
+      );
+    },
+    onAfterAuthUserCreated: (
+      session,
+      authUser, {
+      required transaction,
+    }) async {
+      // e.g. send a welcome email, log for analytics
+    },
+  ),
+);
+```
+
+:::warning
+Both callbacks run inside the same database transaction as the account creation. Throwing an exception inside either callback aborts the sign-up. Wrap external side-effects (email sending, analytics) in `try`/`catch` so a third-party outage does not block new sign-ups.
+:::
+
+## FirebaseIdpConfig parameter reference
+
+| Parameter | Type | Required | Description |
+| --- | --- | --- | --- |
+| `credentials` | `FirebaseServiceAccountCredentials` | Yes | Firebase service account credentials for verifying ID tokens. Can be loaded via `fromJsonString`, `fromJsonFile`, or `fromJson`. When using `FirebaseIdpConfigFromPasswords`, this is loaded automatically from the `firebaseServiceAccountKey` key in `passwords.yaml` or the `SERVERPOD_PASSWORD_firebaseServiceAccountKey` environment variable. |
+| `firebaseAccountDetailsValidation` | `FirebaseAccountDetailsValidation?` | No | Custom validation callback for Firebase account details before allowing sign-in. By default, validates that email is verified when present (phone-only auth is allowed). |
+| `onAfterFirebaseAccountCreated` | `AfterFirebaseAccountCreatedFunction?` | No | Callback invoked after a new Firebase account has been created and linked to an auth user. Receives the session, the created `AuthUserModel`, the `FirebaseAccount`, and the active `Transaction`. Runs inside the same database transaction as account creation, so the `transaction` can be used to perform additional database operations atomically with sign-up. |
+| `clockSkewTolerance` | `Duration` | No | Tolerance for clock skew when validating Firebase ID token timestamps. Defaults to the framework's default clock skew tolerance. |