Fix: Linux AppImage update handling for in-app updating.

StabilityMatrix.Avalonia/ViewModels/Dialogs/UpdateViewModel.cs

@@ -162,6 +162,68 @@ await updateHelper.DownloadUpdate(
             );
         }
 
+        // Handle Linux AppImage update
+        if (Compat.IsLinux && Environment.GetEnvironmentVariable("APPIMAGE") is { } appImage)
+        {
+            try
+            {
+                var updateScriptPath = UpdateHelper.UpdateFolder.JoinFile("update_script.sh").FullPath;
+                var newAppImage = UpdateHelper.ExecutablePath.FullPath;
+
+                var scriptContent = """
+#!/bin/bash
+set -e
+
+# Wait for the process to exit
+while kill -0 "$PID" 2>/dev/null; do
+    sleep 0.5
+done
+
+# Move the new AppImage over the old one
+mv -f "$NEW_APPIMAGE" "$OLD_APPIMAGE"
+chmod +x "$OLD_APPIMAGE"
+
+# Launch the new AppImage detached
+"$OLD_APPIMAGE" > /dev/null 2>&1 &
+disown
+""";
+                await File.WriteAllTextAsync(updateScriptPath, scriptContent);
+
+                File.SetUnixFileMode(
+                    updateScriptPath,
+                    UnixFileMode.UserRead | UnixFileMode.UserWrite | UnixFileMode.UserExecute
+                );
+
+                var startInfo = new System.Diagnostics.ProcessStartInfo
+                {
+                    FileName = "/usr/bin/env",
+                    Arguments = $"bash \"{updateScriptPath}\"",
+                    UseShellExecute = false,
+                    CreateNoWindow = true,
+                };
+
+                startInfo.EnvironmentVariables["PID"] = Environment.ProcessId.ToString();
+                startInfo.EnvironmentVariables["NEW_APPIMAGE"] = newAppImage;
+                startInfo.EnvironmentVariables["OLD_APPIMAGE"] = appImage;
+
+                System.Diagnostics.Process.Start(startInfo);
+
+                App.Shutdown();
+                return;
+            }
+            catch (Exception ex)
+            {
+                logger.LogError(ex, "Failed to execute AppImage update script");
+
+                var dialog = DialogHelper.CreateMarkdownDialog(
+                    "AppImage update script failed. \nCould not replace old AppImage with new version. Please check directory permissions. \nFalling back to standard update process. User intervention required: \nAfter program closes, \nplease move the new AppImage extracted in the '.StabilityMatrixUpdate' hidden directory to the old AppImage overwriting it. \n\nClose this dialog to continue with standard update process.",
+                    Resources.Label_UnexpectedErrorOccurred
+                );
+
+                await dialog.ShowAsync();
+            }
+        }
+
         // Set current version for update messages
         settingsManager.Transaction(
             s => s.UpdatingFromVersion = Compat.AppVersion,