Fix WordDetailModal: Complete localhost:3000 hardcoding cleanup

- Fixed API_URL in WordDetailModal.tsx to use explicit undefined check
- This completes the Docker deployment fix for all frontend components
- Word info modal now works correctly in Docker via nginx proxy
- All 4 locations now use consistent environment-aware URL pattern:
  * api.ts (API client)
  * videoPlaylist.ts (video URLs)
  * VideoUpload.tsx (admin video upload)
  * WordDetailModal.tsx (word info modal)

Fixes: Word info click feature in Docker deployment

Co-Authored-By: Auggie

packages/frontend/src/components/dictionary/WordDetailModal.tsx

@@ -57,7 +57,7 @@ export function WordDetailModal({ term, open, onClose, onAddToSentence }: WordDe
   const imageMedia = term.media?.find(m => m.kind === 'IMAGE' || m.kind === 'ILLUSTRATION');
 
   // Construct full video URL
-  const API_URL = import.meta.env.VITE_API_URL || 'http://localhost:3000';
+  const API_URL = import.meta.env.VITE_API_URL !== undefined ? import.meta.env.VITE_API_URL : 'http://localhost:3000';
   const videoUrl = videoMedia?.url.startsWith('http')
     ? videoMedia.url
     : `${API_URL}${videoMedia?.url}`;