fix(design-system): force type=submit on StyledFormBuilder#submit

The DS::Button default-type-button change in the previous commit
broke every `form.submit "Log in"` callsite because
`StyledFormBuilder#submit` (app/helpers/styled_form_builder.rb)
renders a DS::Button under the hood with no explicit `type:`.

After the default flip, those submit buttons rendered as
`type="button"`, so submitting forms (login, password reset, every
form using `form.submit`) silently no-ops. CI surfaced this via
~30 system tests failing in the `sign_in` helper, which couldn't
get past the login page.

Pin `type: "submit"` on the DS::Button rendered by
`StyledFormBuilder#submit`. The 22 view-level `f.submit` /
`render DS::Button.new(type: :submit, ...)` callers already pass
type explicitly and are unaffected.

app/helpers/styled_form_builder.rb

@@ -89,6 +89,7 @@ class StyledFormBuilder < ActionView::Helpers::FormBuilder
     @template.render(
       DS::Button.new(
         text: value,
+        type: "submit",
         data: (options[:data] || {}).merge({ turbo_submits_with: "Submitting..." }),
         full_width: true
       )