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.

[github-actions]

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[nilsding]

Thanks for your contribution! Some points:

• Try to make it work without relying on an external shell invocation: /bin/bash may not be available on all systems, and the current implementation would break if the AppImage was started from a path such as /home/testing/"; kcalc& "/nextcloud.AppImage
• Instead of using merge commits to update your branch, rebase your work on top of the target branch.
• The DCO trailer is missing from your commit message, please read through contribute/developer-certificate-of-origin and if you can certify it add it as per https://github.com/nextcloud/desktop/blob/master/CONTRIBUTING.md#sign-your-work.

I am also curious about the "Fixes" mentioned in the PR description, did you run into those while implementing the AppImageUpdater class?

[github-actions]

Artifact containing the AppImage: nextcloud-appimage-pr-9269.zip

Digest: sha256:a1cf988be09cd6816a165f04443c5b849d42cbfb487b4bbc996561145023097c

To test this change/fix you can download the above artifact file, unzip it, and run it.

Please make sure to quit your existing Nextcloud app and backup your data.

[mgallien]

see my inline comments
please use auto as much as possible
I will try to test it locally

[derspotter]

I tried to resolve everything in 02a29e7 and kindly ask for a new review.

[derspotter]

Update: pushed a fix to keep updater dialogs in the foreground.
Summary:

• Added helper that waits for an active window, then parents update dialogs to it (WindowModal), with a 2s fallback to ApplicationModal if no window appears. Re-raises dialogs on focus changes and keeps them on top to avoid being hidden behind the setup wizard.\n- Applied to both OCUpdater (NSIS) and AppImageUpdater error dialogs.
• Tested via AppImage build 10006 on Linux: update-failed dialog now appears in the foreground.