feat: implement AppImage auto-updater for Linux

[derspotter]

This PR implements a self-update mechanism for the Linux AppImage.

Changes:

• Added AppImageUpdater class (inherits OCUpdater) to handle AppImage-specific update logic.
• Implemented detection of AppImage environment (APPIMAGE env var).
• Added manual handling of 302 Found redirects to support GitHub Releases downloads.
• Added buffer clearing logic to prevent binary corruption when servers send HTML bodies with redirects.
• Implemented a temporary bash script mechanism to replace the running AppImage file and restart the application.
• Updated Updater::create factory to instantiate AppImageUpdater on Linux when applicable.
• Updated CMakeLists.txt and build-appimage.sh to include the new files and ensure clean builds.

Fixes:

• Fixes issues where the updater would not follow GitHub release redirects.
• Fixes binary corruption issues caused by writing redirect responses to the file.
• Enables seamless updates for AppImage users.