feat: registration field polish, multi-category tags, file uploads, Partner icon

- Restructure field editor dialog: move Options section to bottom with
  divider and subheader, fix delete button with flex layout
- Change tag_category (single string) to tag_categories (JSON array)
  supporting multiple category selection in tag picker fields
- Portal tag picker now groups tags by category with subheaders
- Add generic file upload endpoint (FileUploadService + UploadController)
- Replace email branding logo URL text field with ImageUploadField
- Update Partner crowd type default icon to tabler-affiliate
- Apply changes consistently to both field and template dialogs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/database/factories/RegistrationFormFieldFactory.php

@@ -27,7 +27,7 @@ final class RegistrationFormFieldFactory extends Factory
             'slug' => Str::slug($label),
             'field_type' => RegistrationFieldType::TEXT,
             'options' => null,
-            'tag_category' => null,
+            'tag_categories' => null,
             'is_required' => false,
             'is_portal_visible' => true,
             'is_admin_only' => false,