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/Http/Requests/Api/V1/StoreRegistrationFieldTemplateRequest.php

@@ -43,11 +43,11 @@ final class StoreRegistrationFieldTemplateRequest extends FormRequest
             'options.*' => ['required'],
             'options.*.label' => ['required_if:options.*,array', 'string', 'max:255'],
             'options.*.description' => ['nullable', 'string', 'max:200'],
-            'tag_category' => [
+            'tag_categories' => [
                 $type === RegistrationFieldType::TAG_PICKER ? 'nullable' : 'prohibited',
-                'string',
-                'max:50',
+                'array',
             ],
+            'tag_categories.*' => ['string', 'max:50'],
             'is_required' => ['nullable', 'boolean'],
             'is_filterable' => ['nullable', 'boolean'],
             'is_portal_visible' => ['nullable', 'boolean'],