feat(report-viewer): add searchable combobox for report selection

Replace native select dropdown with Headless UI Combobox component to enable:
- Search/filter functionality for reports
- Better UX with keyboard navigation
- Improved visual feedback for selected items
- Empty state handling for no matches

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

package.json

@@ -34,6 +34,7 @@
     "@aws-sdk/client-s3": "^3.929.0",
     "@electron-toolkit/preload": "^3.0.2",
     "@electron-toolkit/utils": "^4.0.0",
+    "@headlessui/react": "^2.2.9",
     "@tailwindcss/vite": "^4.2.1",
     "@types/js-yaml": "^4.0.9",
     "chromium-bidi": "^15.0.0",