Fix open redirect vulnerability in account login/logout views

[Copilot]

The nextPage parameter in loginView, logoutView, and updateProfile was passed directly to HttpResponseRedirect with no validation, allowing unauthenticated attackers to redirect users to arbitrary external domains (e.g. /account/logout?nextPage=http://evil.example.com).

Changes

• account/views.py: Added _get_safe_next_page() helper using Django's url_has_allowed_host_and_scheme() to validate redirect targets. External URLs fall back to reverse('browser'). Applied to all three affected views.

def _get_safe_next_page(request, param_value):
    if url_has_allowed_host_and_scheme(
        url=param_value,
        allowed_hosts={request.get_host()},
        require_https=request.is_secure(),
    ):
        return param_value
    return reverse('browser')

• tests/test_account.py: New test module covering blocked external redirects and permitted relative redirects for login and logout flows.

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