Disable update prompts when installed from Microsoft Store

[Slashxdd]

Fixes #127

Problem

Users who install via the Microsoft Store are prompted on every launch to update to a GitHub release that isn't published on the Store. The Store manages its own update lifecycle, so these prompts are incorrect and confusing.

Detection

Uses GetCurrentPackageFullName from kernel32.dll — the standard Windows API for detecting packaged (Store) apps. Returns APPMODEL_ERROR_NO_PACKAGE (15700) for non-packaged installs. This is the same approach used by major apps (e.g. VS Code, Spotify) to distinguish Store from direct installs.

[DllImport("kernel32.dll", CharSet = CharSet.Unicode)]
private static extern int GetCurrentPackageFullName(ref int packageFullNameLength, IntPtr packageFullName);

internal static bool IsInstalledFromStore()
{
    int length = 0;
    return GetCurrentPackageFullName(ref length, IntPtr.Zero) != 15700;
}

Changes

File	Change
MainForm.cs	Added IsInstalledFromStore() helper; skip startup CheckForUpdates call for Store installs
AboutForm.cs	Hide "Check for app updates" button for Store installs
SettingsForm.cs	Hide "Check for app updates" checkbox for Store installs

For non-Store installs, behaviour is completely unchanged.