refactor: introduce dedicated identifier types for improved clarity and type safety

frontend/src/routes/DocumentViewerRoute.tsx

@@ -2,8 +2,7 @@ import React, { useEffect } from 'react';
 import { Navigate, useNavigate, useParams } from 'react-router-dom';
 
 import { useAppShell } from '../appShellContext';
-
-type Identifier = string | number;
+import type { Identifier } from '../types/identifiers';
 
 interface DocumentViewerRouteContext {
   previewWorkspaceDocument?: { id?: Identifier } | null;