feat(universal-data-grid): implement UniversalDataGrid component (#611)

Adds a generic, typed enterprise-grade data grid built on
@tanstack/react-table and @tanstack/react-virtual: virtualized rows,
per-column sorting and text filtering, left/right column pinning,
CSV export, and full WAI-ARIA grid keyboard navigation. Wired into
the demo page with a 5,000-row synthetic attendee dataset.

Author: ThatCodeBabe

app/frontend/app/demo/page.tsx

@@ -5,6 +5,7 @@ import MultiSourceDataAggregator from '../../components/MultiSourceDataAggregato
 import SessionTimeoutManager from '../../components/SessionTimeoutManager';
 import { DIDCredentialCard } from '../../components/ui/molecules';
 import ConferenceFloorMap from '../../components/ConferenceFloorMap';
+import UniversalDataGrid, { DEFAULT_ATTENDEES, ATTENDEE_COLUMNS } from '../../components/UniversalDataGrid';
 
 // Demo page for testing both components
 const ComponentDemoPage: React.FC = () => {
@@ -117,6 +118,21 @@ const ComponentDemoPage: React.FC = () => {
           />
         </section>
 
+        {/* UniversalDataGrid Demo */}
+        <section>
+          <h2 className="text-2xl font-semibold text-gray-800 mb-4">
+            UniversalDataGrid Demo
+          </h2>
+          <p className="text-sm text-gray-600 mb-4">
+            5,000 virtualized rows. Click a header to sort, type in a column filter, use the pin icon to stick a column left/right, and use Export CSV. Arrow keys navigate cells; Enter/Space sorts a focused header.
+          </p>
+          <UniversalDataGrid
+            data={DEFAULT_ATTENDEES}
+            columns={ATTENDEE_COLUMNS}
+            getRowId={(row) => row.id}
+          />
+        </section>
+
         {/* MultiSourceDataAggregator Demo */}
         <section>
           <h2 className="text-2xl font-semibold text-gray-800 mb-4">