v3.0.0

Added

• Web app (web_app.py) — Flask-based browser interface. Run locally or
  deploy to any WSGI host. Includes JSON API, per-job progress polling, and a
  shared dark theme matching the desktop GUI.
• Tri-language UI — English (default), Traditional Chinese, Simplified
  Chinese. Switch at runtime; both interfaces share i18n.py.
• Shared engine (core.py) — extracted yt-dlp + ffmpeg logic into a
  UI-agnostic Downloader / Converter pair so the desktop GUI and web app
  reuse the exact same code path.

Changed

• English-first interface — all UI text, log lines, and dialog messages
  now default to English; the previous bilingual labels are replaced by a
  language switcher.
• Cleaner architecture — desktop GUI shrunk by factoring the engine out,
  leaving the file focused on layout + tkinter glue.
• Format string handling — log messages use keyed templates resolved
  through Translator.t(key, **fmt) so adding languages no longer requires
  editing core logic.

Improved

• Cooperative stop now reliably interrupts both downloads and conversions via
  Downloader.stop() / Converter.stop() on the shared engine.
• Progress callbacks now report percent, speed, and ETA as a typed tuple
  instead of stringly-typed config calls.
• format_filesize() extended to GB and keeps the 1 MiB boundary fix.
• Conversion status cells in the desktop tree now re-translate when the
  language is switched mid-session.