Replaced all references to notification.mp3 with hohoho.mp3 in notification-related components and context. Added hohoho.mp3 to the public directory. Introduced a script (download-notification-sound.js) for downloading notification sounds from YouTube using @distube/ytdl-core, which was added as a devDependency. Also added yt-dlp.exe for alternative audio downloading.
Replaces hardcoded production API URLs with localhost defaults for local development in both server and client code. Updates Dockerfile to require API URLs via deployment environment variables. Improves ChatTable to use a batch endpoint for chats and unread counts, with backward compatibility. Adds an env.example file to document required environment variables. Updates next.config.mjs to use environment variables for backend API rewrites and image domains.
