feat: major project restructure - move Django to backend dir and fix critical imports

- Restructure project: moved Django backend to backend/ directory
- Add frontend/ directory for future Next.js application
- Add shared/ directory for common resources
- Fix critical Django import errors:
  - Add missing sys.path modification for apps directory
  - Fix undefined CATEGORY_CHOICES imports in rides module
  - Fix media migration undefined references
  - Remove unused imports and f-strings without placeholders
- Install missing django-environ dependency
- Django server now runs without ModuleNotFoundError
- Update .gitignore and README for new structure
- Add pnpm workspace configuration for monorepo setup

templates/rides/partials/designer_created.html

@@ -1,26 +0,0 @@
-{% if error %}
-    <div class="p-4 mb-4 text-sm text-red-700 bg-red-100 rounded-lg dark:bg-red-200 dark:text-red-800" role="alert">
-        {{ error }}
-    </div>
-{% else %}
-    <div class="p-4 mb-4 text-sm text-green-700 bg-green-100 rounded-lg dark:bg-green-200 dark:text-green-800" role="alert">
-        {% if designer.id %}
-            Designer "{{ designer.name }}" has been created successfully.
-            <script>
-                // Update the designer field in the parent form
-                selectDesigner('{{ designer.id }}', '{{ designer.name }}');
-                // Close the modal
-                document.dispatchEvent(new CustomEvent('close-designer-modal'));
-            </script>
-        {% else %}
-            Your designer submission "{{ designer.name }}" has been sent for review.
-            You will be notified when it is approved.
-            <script>
-                // Close the modal after a short delay
-                setTimeout(() => {
-                    document.dispatchEvent(new CustomEvent('close-designer-modal'));
-                }, 2000);
-            </script>
-        {% endif %}
-    </div>
-{% endif %}