Modularize and streamline billing templates; replace _billing_form.html with reusable modals and shared partials, and update styles and interactions for consistency and clarity. billing, feature management, and forms

Refactored club detail templates to modularize common UI components. Standardized layout, interactions, and styles across admin, billing, and feature cards for consistency and reusability.

controlpanel/mixins.py

@@ -1,5 +1,7 @@
+from django.contrib import messages
 from django.contrib.auth.mixins import UserPassesTestMixin
 from django.http import Http404
+from django.shortcuts import redirect
 
 
 class PlatformStaffRequiredMixin(UserPassesTestMixin):
@@ -38,3 +40,21 @@ class PlatformSuperuserRequiredMixin(PlatformStaffRequiredMixin):
 
     def test_func(self):
         return self.request.user.is_superuser
+
+
+class RedirectOnInvalidMixin:
+    """A form submitted from a modal has nowhere sensible to re-render on error: the page
+    that opened it has already moved on, and the view has no standalone template of its
+    own. Redirect back to ``invalid_redirect_url_name`` instead, with the errors flattened
+    into messages, rather than Django's default of re-rendering ``template_name``.
+    """
+
+    invalid_redirect_url_name = None
+
+    def get_invalid_redirect_kwargs(self):
+        return {}
+
+    def form_invalid(self, form):
+        for error in form.errors.values():
+            messages.error(self.request, " ".join(error))
+        return redirect(self.invalid_redirect_url_name, **self.get_invalid_redirect_kwargs())