fix: custom menu security hardening and code quality improvements

- Add admin menu permission check in CustomPageView (visibility + role)
- Sanitize SVG content with DOMPurify before v-html rendering (XSS prevention)
- Decouple router.go from dto package using anonymous struct
- Consolidate duplicate parseCustomMenuItems into dto.ParseCustomMenuItems
- Enhance menu item validation (count, length, ID uniqueness limits)
- Add audit logging for purchase_subscription and custom_menu_items changes
- Update API contract test to include custom_menu_items field

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/server/api_contract_test.go

@@ -513,7 +513,8 @@ func TestAPIContracts(t *testing.T) {
 					"hide_ccs_import_button": false,
 					"purchase_subscription_enabled": false,
 					"purchase_subscription_url": "",
-					"min_claude_code_version": ""
+					"min_claude_code_version": "",
+					"custom_menu_items": []
 				}
 			}`,
 		},