v2.5 - About dialog & update check

What's new

About dialog + Check for updates. A new About button in the header opens
a themed dialog with:

• the app and ffmpeg versions,
• links to the GitHub project and the website,
• license info (Leike is MIT; bundled ffmpeg is GPLv3) with a View licenses
  button, and
• a manual Check for updates that compares your version against the latest
  GitHub release and offers a download link when a newer one exists.

The update check is manual-only — the app never phones home unless you click the
button.

Downloads

• Windows installer — Leike-Setup.exe (ffmpeg bundled; per-user, no admin
  prompt; clean uninstall).
• Windows portable — Leike-portable-win64.zip (unzip and run; ffmpeg
  bundled).
• Windows standalone — Leike.exe (drop an ffmpeg.exe beside it, or use
  the installer / portable).
• macOS (Apple Silicon) — Leike-macOS-arm64.zip (brew install ffmpeg
  first; first launch: right-click → Open).
• Linux (x86_64) — Leike-linux-x86_64.tar.gz (install ffmpeg with your
  package manager).