feat(observability): add App.Error interface with errorId to app.d.ts

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

frontend/src/app.d.ts

@@ -26,6 +26,11 @@ declare global {
 		interface PageData {
 			user?: User; // Available in $page.data.user
 		}
+
+		interface Error {
+			message: string;
+			errorId?: string;
+		}
 	}
 }