[mob] Include mobile flag during ott (#6906)

## Description
Need this to identify which OTT template to send from backend.
Ref: https://github.com/ente-io/ente/pull/5654
## Tests

mobile/apps/photos/lib/services/account/user_service.dart

@@ -1,5 +1,6 @@
 import 'dart:async';
 import "dart:convert";
+import "dart:io";
 import "dart:math";
 
 import 'package:bip39/bip39.dart' as bip39;
@@ -102,6 +103,7 @@ class UserService {
         data: {
           "email": email,
           "purpose": isChangeEmail ? "change" : purpose ?? "",
+          "mobile": Platform.isIOS || Platform.isAndroid,
         },
       );
       await dialog.hide();

mobile/packages/accounts/lib/services/user_service.dart

@@ -1,5 +1,6 @@
 import 'dart:async';
 import "dart:convert";
+import "dart:io";
 import "dart:math";
 
 import 'package:bip39/bip39.dart' as bip39;
@@ -88,6 +89,7 @@ class UserService {
         data: {
           "email": email,
           "purpose": isChangeEmail ? "change" : purpose ?? "",
+          "mobile": Platform.isIOS || Platform.isAndroid,
         },
       );
       await dialog.hide();