feat(admin): add custom admin site that reuses the allauth login

[suricactus]

Since login is a quite a critical part of every application, it would be great if it is always done via a single entrypoint. However, currently the login is handled either via allauth login, or via Django admin login for django admin.

Or in other words, there are accounts/login and admin/login. Suboptimal and fixed in this PR.

What is more, adding a custom admin site for QFieldCloud will allow us to do other "magical" stuff.

In a follow-up PR we need to clean-up the current available configuration on the admin template and provide the same on the allauth template.

Also solves #1259

[duke-nyuki]

Task linked: QF-6412 Use only one login page from allauth

[suricactus]

@tdrivas when I ran the test locally, I couldn't manage to reproduce, any ideas what is wrong?

*[QF-6412-custom_admin][~/work/opengisch/qfieldcloud-private]$ docker compose --env-file .env.test exec app python manage.py test --keepdb qfieldcloud.authentication.tests.test_authentication.QfcTestCase                                         
{"request_id":"none","ts":"2025-07-24T22:30:03.823696","level":"INFO","name":"axes.apps","message":"AXES: BEGIN version 7.0.2, blocking by username","filename":"apps.py","lineno":53,"thread":134119918529408,"source":"app"}
Found 9 test(s).
Using existing test database for alias 'default'...
System check identified no issues (0 silenced).
./usr/local/lib/python3.10/site-packages/allauth/account/app_settings.py:117: UserWarning: app_settings.AUTHENTICATION_METHOD is deprecated, use: app_settings.LOGIN_METHODS
  warnings.warn(
........
----------------------------------------------------------------------
Ran 9 tests in 47.713s

OK
Preserving test database for alias 'default'...

[tdrivas]

@tdrivas when I ran the test locally, I couldn't manage to reproduce, any ideas what is wrong?

*[QF-6412-custom_admin][~/work/opengisch/qfieldcloud-private]$ docker compose --env-file .env.test exec app python manage.py test --keepdb qfieldcloud.authentication.tests.test_authentication.QfcTestCase                                         
{"request_id":"none","ts":"2025-07-24T22:30:03.823696","level":"INFO","name":"axes.apps","message":"AXES: BEGIN version 7.0.2, blocking by username","filename":"apps.py","lineno":53,"thread":134119918529408,"source":"app"}
Found 9 test(s).
Using existing test database for alias 'default'...
System check identified no issues (0 silenced).
./usr/local/lib/python3.10/site-packages/allauth/account/app_settings.py:117: UserWarning: app_settings.AUTHENTICATION_METHOD is deprecated, use: app_settings.LOGIN_METHODS
  warnings.warn(
........
----------------------------------------------------------------------
Ran 9 tests in 47.713s

OK
Preserving test database for alias 'default'...

Hey,

I did manage to reproduce by following the steps below:

1. use of .env.example
2. Build and start the full stack
3. Collect static

docker compose run --env-file .env.example --rm app python manage.py collectstatic --noinput

4. Run the single failing test with the same flags

docker compose --env-file .env.example run --rm app \
  python manage.py test --keepdb \
    qfieldcloud.authentication.tests.test_authentication.QfcTestCase.test_login_with_session_case_insensitive

[suricactus]

@tdrivas thanks, managed to reproduce with the provided steps!

[suricactus]

Do not merge, as the login page being used now is really ugly.