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/app/Models/RegistrationFormField.php

@@ -33,7 +33,7 @@ final class RegistrationFormField extends Model
         'slug',
         'field_type',
         'options',
-        'tag_category',
+        'tag_categories',
         'is_required',
         'is_portal_visible',
         'is_admin_only',
@@ -48,6 +48,7 @@ final class RegistrationFormField extends Model
         return [
             'field_type' => RegistrationFieldType::class,
             'options' => 'array',
+            'tag_categories' => 'array',
             'is_required' => 'boolean',
             'is_portal_visible' => 'boolean',
             'is_admin_only' => 'boolean',