Use env variable for max file size limit

- Frontend: read VITE_MAX_FILE_SIZE_MB from env
- Update .env.example files with limits

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontend/.env.example

@@ -1,2 +1,3 @@
 VITE_API_URL=http://localhost:4001
 VITE_WS_URL=ws://localhost:4001
+VITE_MAX_FILE_SIZE_MB=10