Fix updater file overwrite mode

[LucHeart]

Summary

• ensure the updater overwrites the downloaded installer using FileMode.Create with write access

Testing

• dotnet publish Desktop/Desktop.csproj -c Release-Photino -o ./publish/Photino-Linux (fails: dotnet is not installed in the container)

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https://chatgpt.com/codex/tasks/task_e_68f752b83fd08327ab064b017f7665c6

[Copilot]

Pull Request Overview

This PR fixes a file handling issue in the updater service by ensuring downloaded installers are properly overwritten. The change replaces FileMode.OpenOrCreate with FileMode.Create and explicitly specifies write-only access with no file sharing to prevent file access conflicts during updates.

Key Changes:

• Modified file stream creation in the updater to use FileMode.Create instead of FileMode.OpenOrCreate
• Added explicit FileAccess.Write and FileShare.None parameters for safer file handling

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