Fix empty allauth forms, and lay out the 2FA page

The 2FA code input was not invisible -- it was absent, along with the fields of
every other allauth form except login.

Cause: the `fields` element passed `attrs.exclude` straight into a filter. On a
page that never sets it, resolving a filter *argument* raises
VariableDoesNotExist; Django rescues that for the main variable of an expression
but not for a filter argument, and {% if %} then swallows it and reads the
condition as false. So every field was skipped. Login was the one page that
passes `exclude`, which is exactly why it kept working and hid the damage.
`exclude` is now pinned to a real variable first, with tests that render the
login, signup and password-reset forms and assert their inputs exist.

Two dangling buttons fixed while in here: `elements/form.html` dropped the `id`
attribute, so the out-of-band forms allauth generates (webauthn_form,
logout-from-stage) had no id for a button's `form` attribute to point at. "Use a
security key" submitted nothing.

Layout: the code is a daisyUI otp field, Cancel sits beside Sign In as a plain
button, both gain icons, and "Use a security key" becomes an accent button.

The otp boxes yield once more than six characters are typed. allauth accepts a
TOTP code (6) *or* a recovery code (8) in this one field, so hard-boxing it to
six would have locked out every recovery code.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

templates/allauth/elements/form.html

@@ -1,9 +1,18 @@
 {% load allauth %}
+
+{% comment %}
+    `id` is not decorative: allauth renders out-of-band forms (webauthn, logout-from-stage)
+    and points buttons at them with the HTML `form` attribute. Drop the id and those
+    buttons submit nothing.
+{% endcomment %}
 <form method="{{ attrs.method }}"
+      {% if attrs.id %}id="{{ attrs.id }}"{% endif %}
       {% if attrs.action %}action="{{ attrs.action }}"{% endif %}
       class="space-y-2">
     {% slot body %}{% endslot %}
-    <div class="card-actions justify-end pt-3">
-        {% slot actions %}{% endslot %}
-    </div>
+    {% if not attrs.no_visible_fields %}
+        <div class="card-actions justify-end pt-3">
+            {% slot actions %}{% endslot %}
+        </div>
+    {% endif %}
 </form>